literature review search parameters

How to undertake a literature search: a step-by-step guide

Affiliation.

• 1 Literature Search Specialist, Library and Archive Service, Royal College of Nursing, London.
• PMID: 32279549
• DOI: 10.12968/bjon.2020.29.7.431

Undertaking a literature search can be a daunting prospect. Breaking the exercise down into smaller steps will make the process more manageable. This article suggests 10 steps that will help readers complete this task, from identifying key concepts to choosing databases for the search and saving the results and search strategy. It discusses each of the steps in a little more detail, with examples and suggestions on where to get help. This structured approach will help readers obtain a more focused set of results and, ultimately, save time and effort.

Keywords: Databases; Literature review; Literature search; Reference management software; Research questions; Search strategy.

• Databases, Bibliographic*
• Information Storage and Retrieval / methods*
• Nursing Research
• Review Literature as Topic*

• En español – ExME
• Em português – EME

Conducting a systematic literature search

Posted on 18th July 2017 by Saul Crandon

Systematic reviews sit amongst the top of the evidence hierarchy. This is all well and good, provided they are conducted appropriately . To ensure a review can provide high quality, reliable evidence, they must be completed meticulously, following reporting guidelines such as the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA)  and the Cochrane Handbook (1, 2) A key element of this is a robust search strategy.

Systematic reviews take existing literature and synthesise it, either qualitatively or quantitatively. In order to be ‘systematic’ and minimise selection bias, the search must be comprehensive, encompassing all of the relevant research. Bias will exist if the authors fail to include ALL of the pertinent research. “How can you be sure your conclusions are true if you have not explored all of the evidence?”

Of course, some articles may be excluded (for instance if they are methodologically flawed) once you come to screening the evidence, especially if this is a meta-analysis. However, the authors must ensure that initially all of the relevant evidence is captured by the search.

Performing an in-depth search is often overwhelming for authors setting out with a systematic review. It can seem confusing or difficult and is often substituted for a more manageable, less rigorous search. The following checklist should provide a clear framework for those wanting to ensure their search is truly systematic.

All illustrative example of database searching will be performed on Ovid Medline. Please note that this is a basic worked example to illustrate some of the key principles involved in searching. Using the search outlined below, you are likely to pick up key studies. However, to minimise your chances of missing relevant articles then do consultant a librarian who will be able to assist you with advanced search techniques. This is the best way to maximise sensitivity and reduce your risk of missing relevant articles.

BEFORE THE SEARCH

Firstly, it is important that systematic review protocols (including search strategies) are prospectively registered to prevent deviation from the intended methods and minimise bias. A popular registration database is the International Prospective Register of Systematic Reviews (PROSPERO).

Also, it is a good idea to carry out a search for any similar systematic reviews. It may be pointless to repeat an existing systematic review. Although if an existing systematic review is outdated or features flawed methodology, then a new systematic review on that topic can be justified.

Defining the research question

Before a search can be performed, it is crucial that the research question is explicitly defined. There are many ways to do this, but the most common method is to divide your question according to the Patients, Interventions, Comparisons and Outcomes (PICO) model.

Patients: Which patient population do you want to explore? Adults? Elderly? Paediatrics? Males? Females? Certain ethnicities? Inpatients? Community patients? Certain co-morbidities?

Interventions: What intervention(s) do you want to explore? A certain medication? A certain surgical procedure?

Comparisons: What are you going to compare the intervention(s) against? (Some reviews may not have a comparison so this section is optional). Are you going to compare the intervention with no intervention? A control? Usual treatment? A different medication? A different surgical procedure?

Outcomes: What outcomes do you want to explore? Mortality rate? Morbidity or the development of a certain disease? Change in baseline parameters such as blood pressure, weight or cholesterol?

Defining the parameters of the search

Once a well-defined research question has been established, it is important to outline where you will search for the evidence. Systematic searches should aim to search as many different sources as possible. This can be broken down into the following:

Sources Online databases* (this will be the major area for medical literature searches) Books/physical literature Grey literature* (this refers to unpublished material/published in a non-commercial form) Ongoing trials ( clinicaltrials.gov )

Types of articles Randomized controlled trials (RCTs) Cohort studies Case-control studies Case series studies Case reports Other

Dates How far back do you wish to explore? Perhaps the intervention you want to investigate was only developed in the last 10 years

Languages English Other languages too? Ideally you should be willing to translate these in order to fully understand their findings

Subjects Humans Animals Other

*The following databases are useful for clinical systematic reviews:

PsychINFO – key database for mental health literature MEDLINE – large medical database EMBASE – large medical database SCOPUS – includes many scientific disciplines Cochrane Library – high-quality evidence Web of Science – includes many scientific disciplines CINAHL – includes biomedicine, healthcare, nursing and allied health articles

*Examples of grey literature databases include: OpenGrey Copac

THE SEARCH ITSELF

Searching online databases.

When searching online databases, the terms and their synonyms for each of the components of the PICO model must be written out, including abbreviations. It is also important to use alternate spellings and word endings. This can be done using a number of strategies within the database:

Firstly, truncation involves putting an asterisk (*) in a word with a variable ending e.g. toxic* will search for toxic, toxicity and toxicology.

Secondly, using a question mark (?) in place of a single letter for words that have alternate spellings (such as American vs British English) will search for both spellings, e.g. p?ediatric will search for both paediatric and pediatric. The question mark replaces a single character or no character, allowing you to pick up alternate spellings. It should be noted that these terms can vary between databases, and so should always be checked prior to conducting a search.

You should also search for hyphenated terms with and without the hyphen as different authors may title their work differently.

An example to illustrate a search in Ovid Medline:

Let’s imagine we want to conduct a systematic review to assess the effect of primary percutaneous coronary intervention (PCI) versus streptokinase on mortality in myocardial infarction (MI) patients. (Clicking on the screenshots below will enlarge them). 

Patients: myocardial infarction, MI, myocardial necrosis, heart attack

As you can see, each line represents a synonymous term that is searched for, with the corresponding number of articles found in the results column. These terms can be combined using ‘OR’ to ensure the database captures all of the articles relating to these terms.

For the final screenshot, all of the groups of terms are combined with ‘AND’ to ensure the database only displays results that are related to the combination of terms (i.e. about PCI and streptokinase in MI patients). In this example, it yielded a total of 254 results.

Further refining or increasing your results

Medical Subject Headings (or MeSH terms) are terms predefined by the database using human indexers in concordance with thorough protocols. MeSH terms encompass ‘Headings’, ‘Subheadings’, ‘Supplementary Concept Records’ and ‘Publication Characteristics’. Further definitions of these categories and relevant examples can be found on the National Library of Medicine’s website ( https://www.nlm.nih.gov/bsd/disted/meshtutorial/introduction/04.html ). The same website also provides detailed descriptions of how the MeSH system is structured and how this can be searched to further refine or expand your literature search. Using Medline as an example, you must select ‘Map term to subject heading’ to search for MeSH terms.’

If your search yields no results then it is important to re-evaluate your PICO model and broaden the search terms. Conversely, if the results are very expansive this will take considerable effort to screen the relevant information. It may be worth narrowing the search terms in order to achieve a more focused search.

AFTER THE SEARCH

Any more studies to include.

Once the formal search has been completed, you must now remove the duplicates and screen the titles/abstracts of the remaining results. This is a large task in itself, and information on how this can be done is beyond the scope of this blog.

Once competed, a further exploration for relevant studies can begin. There are a number of strategies that should be carried out at this stage.

• Citation tracking – This involves searching for studies that have referenced the included studies in their work, and as such, may be relevant to your systematic review. Most databases will have an integrated tool to perform this.
• Manual reference searching – Once you have finalised a list of applicable studies, you should go through their references individually and search for additional relevant studies.
• Contacting authors – Finally, authors of the included studies can be contacted to search for further results that may be pertinent to your review, or to ensure your search strategy has not missed any of their other work which may be useful.

At this point it is crucial that you finalise your list of included studies and document your search well so that you can refer to this in the future and during the rest of the review process.

Your SYSTEMATIC literature search is now complete!

Congratulations! If you have followed the various steps in this article, you are well on your way to completing a comprehensive systematic review! If you need further help, consult your librarian.

• Higgins JPT, Green S (editors).  Cochrane Handbook for Systematic Reviews of Interventions  Version 5.1.0 [updated March 2011]. The Cochrane Collaboration, 2011. Available from cochrane-handbook.org .
• Moher D, Liberati A, Tetzlaff J, Altman DG, The PRISMA Group (2009).  Preferred Reporting Items for Systematic Reviews and Meta-Analyses: The PRISMA Statement.  PLoS Med 6(7): e1000097. doi:10.1371/journal.pmed1000097

Saul Crandon

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As I understand your text you suggest using all four categories of PICO in your search. I would argue that doing that is highly unusual in systematic reviews, in fact I can’t remember ever seeing it. I myself rarely use more than population and intervention in my searches.

Apologies Saul, I made a typo in spelling your name!

Great post. I would just emphasize that (as Paul says) it is a ‘strategy’ not a set approach. Web sites and web searches may, for example, be crucial sources for some review questions. See: G. Brunton et al ‘Finding relevant studies’ in Gough et al (2017) ‘Introduction to Systematic Reviews’ 2nd Edn. Sage Publications.

I would advise using both thesaurus terms and natural language terms simultaneously, rather than, as you seem to advocate, only including thesaurus terms to refine results. It isn’t quite correct to say that controlled vocabulary terms are ‘created by the database itself’. In the case of MeSH, these are assigned by human indexers, following a well-described procedure. It is perhaps also worth pointing out that different databases use different thesauri. One criterion by which search strategies can be judged is how well the searcher has translated terms between databases.

And, if you use only e.g. MeSH term you will miss new studies that haven’t been indexed yet.

Your blogs are excellent!

Hi Saul, Great introduction to literature searching. Readers may also be interested in “Grey Matters: a practical tool for searching health-related grey literature,” a comprehensive tool developed by health librarians at CADTH. Grey Matters and it’s companion user guide are available free of charge at http://www.cadth.ca/resources/finding-evidence/grey-matters .

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• How to Write a Literature Review | Guide, Examples, & Templates

How to Write a Literature Review | Guide, Examples, & Templates

Published on January 2, 2023 by Shona McCombes . Revised on September 11, 2023.

What is a literature review? A literature review is a survey of scholarly sources on a specific topic. It provides an overview of current knowledge, allowing you to identify relevant theories, methods, and gaps in the existing research that you can later apply to your paper, thesis, or dissertation topic .

There are five key steps to writing a literature review:

• Search for relevant literature
• Evaluate sources
• Identify themes, debates, and gaps
• Outline the structure
• Write your literature review

A good literature review doesn’t just summarize sources—it analyzes, synthesizes , and critically evaluates to give a clear picture of the state of knowledge on the subject.

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Table of contents

What is the purpose of a literature review, examples of literature reviews, step 1 – search for relevant literature, step 2 – evaluate and select sources, step 3 – identify themes, debates, and gaps, step 4 – outline your literature review’s structure, step 5 – write your literature review, free lecture slides, other interesting articles, frequently asked questions, introduction.

• Quick Run-through
• Step 1 & 2

When you write a thesis , dissertation , or research paper , you will likely have to conduct a literature review to situate your research within existing knowledge. The literature review gives you a chance to:

• Demonstrate your familiarity with the topic and its scholarly context
• Develop a theoretical framework and methodology for your research
• Position your work in relation to other researchers and theorists
• Show how your research addresses a gap or contributes to a debate
• Evaluate the current state of research and demonstrate your knowledge of the scholarly debates around your topic.

Writing literature reviews is a particularly important skill if you want to apply for graduate school or pursue a career in research. We’ve written a step-by-step guide that you can follow below.

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See an example

Writing literature reviews can be quite challenging! A good starting point could be to look at some examples, depending on what kind of literature review you’d like to write.

• Example literature review #1: “Why Do People Migrate? A Review of the Theoretical Literature” ( Theoretical literature review about the development of economic migration theory from the 1950s to today.)
• Example literature review #2: “Literature review as a research methodology: An overview and guidelines” ( Methodological literature review about interdisciplinary knowledge acquisition and production.)
• Example literature review #3: “The Use of Technology in English Language Learning: A Literature Review” ( Thematic literature review about the effects of technology on language acquisition.)
• Example literature review #4: “Learners’ Listening Comprehension Difficulties in English Language Learning: A Literature Review” ( Chronological literature review about how the concept of listening skills has changed over time.)

You can also check out our templates with literature review examples and sample outlines at the links below.

Download Word doc Download Google doc

Before you begin searching for literature, you need a clearly defined topic .

If you are writing the literature review section of a dissertation or research paper, you will search for literature related to your research problem and questions .

Make a list of keywords

Start by creating a list of keywords related to your research question. Include each of the key concepts or variables you’re interested in, and list any synonyms and related terms. You can add to this list as you discover new keywords in the process of your literature search.

• Social media, Facebook, Instagram, Twitter, Snapchat, TikTok
• Body image, self-perception, self-esteem, mental health
• Generation Z, teenagers, adolescents, youth

Search for relevant sources

Use your keywords to begin searching for sources. Some useful databases to search for journals and articles include:

• Your university’s library catalogue
• Google Scholar
• Project Muse (humanities and social sciences)
• Medline (life sciences and biomedicine)
• EconLit (economics)
• Inspec (physics, engineering and computer science)

You can also use boolean operators to help narrow down your search.

Make sure to read the abstract to find out whether an article is relevant to your question. When you find a useful book or article, you can check the bibliography to find other relevant sources.

You likely won’t be able to read absolutely everything that has been written on your topic, so it will be necessary to evaluate which sources are most relevant to your research question.

For each publication, ask yourself:

• What question or problem is the author addressing?
• What are the key concepts and how are they defined?
• What are the key theories, models, and methods?
• Does the research use established frameworks or take an innovative approach?
• What are the results and conclusions of the study?
• How does the publication relate to other literature in the field? Does it confirm, add to, or challenge established knowledge?
• What are the strengths and weaknesses of the research?

Make sure the sources you use are credible , and make sure you read any landmark studies and major theories in your field of research.

You can use our template to summarize and evaluate sources you’re thinking about using. Click on either button below to download.

Take notes and cite your sources

As you read, you should also begin the writing process. Take notes that you can later incorporate into the text of your literature review.

It is important to keep track of your sources with citations to avoid plagiarism . It can be helpful to make an annotated bibliography , where you compile full citation information and write a paragraph of summary and analysis for each source. This helps you remember what you read and saves time later in the process.

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To begin organizing your literature review’s argument and structure, be sure you understand the connections and relationships between the sources you’ve read. Based on your reading and notes, you can look for:

• Trends and patterns (in theory, method or results): do certain approaches become more or less popular over time?
• Themes: what questions or concepts recur across the literature?
• Debates, conflicts and contradictions: where do sources disagree?
• Pivotal publications: are there any influential theories or studies that changed the direction of the field?
• Gaps: what is missing from the literature? Are there weaknesses that need to be addressed?

This step will help you work out the structure of your literature review and (if applicable) show how your own research will contribute to existing knowledge.

• Most research has focused on young women.
• There is an increasing interest in the visual aspects of social media.
• But there is still a lack of robust research on highly visual platforms like Instagram and Snapchat—this is a gap that you could address in your own research.

There are various approaches to organizing the body of a literature review. Depending on the length of your literature review, you can combine several of these strategies (for example, your overall structure might be thematic, but each theme is discussed chronologically).

Chronological

The simplest approach is to trace the development of the topic over time. However, if you choose this strategy, be careful to avoid simply listing and summarizing sources in order.

Try to analyze patterns, turning points and key debates that have shaped the direction of the field. Give your interpretation of how and why certain developments occurred.

If you have found some recurring central themes, you can organize your literature review into subsections that address different aspects of the topic.

For example, if you are reviewing literature about inequalities in migrant health outcomes, key themes might include healthcare policy, language barriers, cultural attitudes, legal status, and economic access.

Methodological

If you draw your sources from different disciplines or fields that use a variety of research methods , you might want to compare the results and conclusions that emerge from different approaches. For example:

• Look at what results have emerged in qualitative versus quantitative research
• Discuss how the topic has been approached by empirical versus theoretical scholarship
• Divide the literature into sociological, historical, and cultural sources

Theoretical

A literature review is often the foundation for a theoretical framework . You can use it to discuss various theories, models, and definitions of key concepts.

You might argue for the relevance of a specific theoretical approach, or combine various theoretical concepts to create a framework for your research.

Like any other academic text , your literature review should have an introduction , a main body, and a conclusion . What you include in each depends on the objective of your literature review.

The introduction should clearly establish the focus and purpose of the literature review.

Depending on the length of your literature review, you might want to divide the body into subsections. You can use a subheading for each theme, time period, or methodological approach.

As you write, you can follow these tips:

• Summarize and synthesize: give an overview of the main points of each source and combine them into a coherent whole
• Analyze and interpret: don’t just paraphrase other researchers — add your own interpretations where possible, discussing the significance of findings in relation to the literature as a whole
• Critically evaluate: mention the strengths and weaknesses of your sources
• Write in well-structured paragraphs: use transition words and topic sentences to draw connections, comparisons and contrasts

In the conclusion, you should summarize the key findings you have taken from the literature and emphasize their significance.

When you’ve finished writing and revising your literature review, don’t forget to proofread thoroughly before submitting. Not a language expert? Check out Scribbr’s professional proofreading services !

This article has been adapted into lecture slides that you can use to teach your students about writing a literature review.

Scribbr slides are free to use, customize, and distribute for educational purposes.

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If you want to know more about the research process , methodology , research bias , or statistics , make sure to check out some of our other articles with explanations and examples.

• Sampling methods
• Simple random sampling
• Stratified sampling
• Cluster sampling
• Likert scales
• Reproducibility

 Statistics

• Null hypothesis
• Statistical power
• Probability distribution
• Effect size
• Poisson distribution

Research bias

• Optimism bias
• Cognitive bias
• Implicit bias
• Hawthorne effect
• Anchoring bias
• Explicit bias

A literature review is a survey of scholarly sources (such as books, journal articles, and theses) related to a specific topic or research question .

It is often written as part of a thesis, dissertation , or research paper , in order to situate your work in relation to existing knowledge.

There are several reasons to conduct a literature review at the beginning of a research project:

• To familiarize yourself with the current state of knowledge on your topic
• To ensure that you’re not just repeating what others have already done
• To identify gaps in knowledge and unresolved problems that your research can address
• To develop your theoretical framework and methodology
• To provide an overview of the key findings and debates on the topic

Writing the literature review shows your reader how your work relates to existing research and what new insights it will contribute.

The literature review usually comes near the beginning of your thesis or dissertation . After the introduction , it grounds your research in a scholarly field and leads directly to your theoretical framework or methodology .

A literature review is a survey of credible sources on a topic, often used in dissertations , theses, and research papers . Literature reviews give an overview of knowledge on a subject, helping you identify relevant theories and methods, as well as gaps in existing research. Literature reviews are set up similarly to other  academic texts , with an introduction , a main body, and a conclusion .

An  annotated bibliography is a list of  source references that has a short description (called an annotation ) for each of the sources. It is often assigned as part of the research process for a  paper .  

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How to write a search strategy for your systematic review

Home | Blog | How To | How to write a search strategy for your systematic review

Practical tips to write a search strategy for your systematic review

With a great review question and a clear set of eligibility criteria already mapped out, it’s now time to plan the search strategy. The medical literature is vast. Your team plans a thorough and methodical search, but you also know that resources and interest in the project are finite. At this stage it might feel like you have a mountain to climb.

The bottom line? You will have to sift through some irrelevant search results to find the studies that you need for your review. Capturing a proportion of irrelevant records in your search is necessary to ensure that it identifies as many relevant records as possible. This is the trade-off of precision versus sensitivity and, because systematic reviews aim to be as comprehensive as possible, it is best to favour sensitivity – more is more.

By now, the size of this task might be sounding alarm bells. The good news is that a range of techniques and web-based tools can help to make searching more efficient and save you time. We’ll look at some of them as we walk through the four main steps of searching for studies:

• Decide where to search
• Write and refine the search
• Run and record the search
• Manage the search results

Searching is a specialist discipline and the information given here is not intended to replace the advice of a skilled professional. Before we look at each of the steps in turn, the most important systematic reviewer pro-tip for searching is:

 Pro Tip – Talk to your librarian and do it early!

1. decide where to search .

It’s important to come up with a comprehensive list of sources to search so that you don’t miss anything potentially relevant. In clinical medicine, your first stop will likely be the databases MEDLINE , Embase , and CENTRAL . Depending on the subject of the review, it might also be appropriate to run the search in databases that cover specific geographical regions or specialist areas, such as traditional Chinese medicine.

In addition to these databases, you’ll also search for grey literature (essentially, research that was not published in journals). That’s because your search of bibliographic databases will not find relevant information if it is part of, for example:

• a trials register
• a study that is ongoing
• a thesis or dissertation
• a conference abstract.

Over-reliance on published data introduces bias in favour of positive results. Studies with positive results are more likely to be submitted to journals, published in journals, and therefore indexed in databases. This is publication bias and systematic reviews seek to minimise its effects by searching for grey literature.

2. Write and refine the search 

Search terms are derived from key concepts in the review question and from the inclusion and exclusion criteria that are specified in the protocol or research plan.

Keywords will be searched for in the title or abstract of the records in the database. They are often truncated (for example, a search for therap* to find therapy, therapies, therapist). They might also use wildcards to allow for spelling variants and plurals (for example, wom#n to find woman and women). The symbols used to perform truncation and wildcard searches vary by database.

Index terms  

Using index terms such as MeSH and Emtree in a search can improve its performance. Indexers with subject area expertise work through databases and tag each record with subject terms from a prespecified controlled vocabulary.

This indexing can save review teams a lot of time that would otherwise be spent sifting through irrelevant records. Using index terms in your search, for example, can help you find the records that are actually about the topic of interest (tagged with the index term) but ignore those that contain only a brief mention of it (not tagged with the index term).

Indexers assign terms based on a careful read of each study, rather than whether or not the study contains certain words. So the index terms enable the retrieval of relevant records that cannot be captured by a simple search for the keyword or phrase.

Use a combination

Relying solely on index terms is not advisable. Doing so could miss a relevant record that for some reason (indexer’s judgment, time lag between a record being listed in a database and being indexed) has not been tagged with an index term that would enable you to retrieve it. Good search strategies include both index terms and keywords.

Let’s see how this works in a real review! Figure 2 shows the search strategy for the review ‘Wheat flour fortification with iron and other micronutrients for reducing anaemia and improving iron status in populations’. This strategy combines index terms and keywords using the Boolean operators AND, OR, and NOT. OR is used first to reach as many records as possible before AND and NOT are used to narrow them down.

• Lines 1 and 2: contain MeSH terms (denoted by the initial capitals and the slash at the end).
• Line 3: contains truncated keywords (‘tw’ in this context is an instruction to search the title and abstract fields of the record).
• Line 4: combines the three previous lines using Boolean OR to broaden the search.
• Line 11: combines previous lines using Boolean AND to narrow the search.
• Lines 12 and 13: further narrow the search using Boolean NOT to exclude records of studies with no human subjects.

Writing a search strategy is an iterative process. A good plan is  to try out a new strategy and check that it has picked up the key studies that you would expect it to find based on your existing knowledge of the topic area. If it hasn’t, you can explore the reasons for this, revise the strategy, check it for errors, and try it again!

3. Run and record the search

Because of the different ways that individual databases are structured and indexed, a separate search strategy is needed for each database. This adds complexity to the search process, and it is important to keep a careful record of each search strategy as you run it. Search strategies can often be saved in the databases themselves, but it is a good idea to keep an offline copy as a back-up; Covidence allows you to store your search strategies online in your review settings.

The reporting of the search will be included in the methods section of your review and should follow the PRISMA guidelines. You can download a flow diagram from PRISMA’s website to help you log the number of records retrieved from the search and the subsequent decisions about the inclusion or exclusion of studies. The PRISMA-S extension provides guidance on reporting literature searches.

It is very important that search strategies are reproduced in their entirety (preferably using copy and paste to avoid typos) as part of the published review so that they can be studied and replicated by other researchers. Search strategies are often made available as an appendix because they are long and might otherwise interrupt the flow of the text in the methods section.

4. Manage the search results 

Once the search is done and you have recorded the process in enough detail to write up a thorough description in the methods section, you will move on to screening the results. This is an exciting stage in any review because it’s the first glimpse of what the search strategies have found. A large volume of results may be daunting but your search is very likely to have captured some irrelevant studies because of its high sensitivity, as we have already seen. Fortunately, it will be possible to exclude many of these irrelevant studies at the screening stage on the basis of the title and abstract alone 😅.

Search results from multiple databases can be collated in a single spreadsheet for screening. To benefit from process efficiencies, time-saving and easy collaboration with your team, you can import search results into a specialist tool such as Covidence. A key benefit of Covidence is that you can track decisions made about the inclusion or exclusion of studies in a simple workflow and resolve conflicting decisions quickly and transparently. Covidence currently supports three formats for file imports of search results:

• EndNote XML
• PubMed text format
• RIS text format

If you’d like to try this feature of Covidence but don’t have any data yet, you can download some ready-made sample data .

And you’re done!

There is a lot to think about when planning a search strategy. With practice, expert help, and the right tools your team can complete the search process with confidence.

This blog post is part of the Covidence series on how to write a systematic review.

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[1] Witt  KG, Hetrick  SE, Rajaram  G, Hazell  P, Taylor Salisbury  TL, Townsend  E, Hawton  K. Pharmacological interventions for self‐harm in adults . Cochrane Database of Systematic Reviews 2020, Issue 12. Art. No.: CD013669. DOI: 10.1002/14651858.CD013669.pub2. Accessed 02 February 2021

Laura Mellor. Portsmouth, UK

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• UConn Library
• Literature Review: The What, Why and How-to Guide
• Strategies to Find Sources

Literature Review: The What, Why and How-to Guide — Strategies to Find Sources

• Getting Started
• Introduction
• How to Pick a Topic
• Evaluating Sources & Lit. Reviews
• Tips for Writing Literature Reviews
• Writing Literature Review: Useful Sites
• Citation Resources
• Other Academic Writings

The Research Process

Planning : Before searching for articles or books, brainstorm to develop keywords that better describe your research question.

Searching : While searching, take note of what other keywords are used to describe your topic, and use them to conduct additional searches

     ♠ Most articles include a keyword section

     ♠ Key concepts may change names throughout time so make sure to check for variations

Organizing : Start organizing your results by categories/key concepts or any organizing principle that make sense for you . This will help you later when you are ready to analyze your findings

Analyzing : While reading, start making notes of key concepts and commonalities and disagreement among the research articles you find.

♠ Create a spreadsheet  to record what articles you are finding useful and why.

♠ Create fields to write summaries of articles or quotes for future citing and paraphrasing .

Writing : Synthesize your findings. Use your own voice to explain to your readers what you learned about the literature on your topic. What are its weaknesses and strengths? What is missing or ignored?

Repeat : At any given time of the process, you can go back to a previous step as necessary.

Advanced Searching

All databases have Help pages that explain the best way to search their product. When doing literature reviews, you will want to take advantage of these features since they can facilitate not only finding the articles that you really need but also controlling the number of results and how relevant they are for your search. The most common features available in the advanced search option of databases and library online catalogs are:

• Boolean Searching (AND, OR, NOT): Allows you to connect search terms in a way that can either limit or expand your search results 
• Proximity Searching (N/# or W/#): Allows you to search for two or more words that occur within a specified number of words (or fewer) of each other in the database
• Limiters/Filters : These are options that let you control what type of document you want to search: article type, date, language, publication, etc.
• Question mark (?) or a pound sign (#) for wildcard: Used for retrieving alternate spellings of a word: colo?r will retrieve both the American spelling "color" as well as the British spelling "colour." 
• Asterisk (*) for truncation: Used for retrieving multiple forms of a word: comput* retrieves computer, computers, computing, etc.

Want to keep track of updates to your searches? Create an account in the database to receive an alert when a new article is published that meets your search parameters!

• EBSCOhost Advanced Search Tutorial Tips for searching a platform that hosts many library databases
• Library's General Search Tips Check the Search tips to better used our library catalog and articles search system
• ProQuest Database Search Tips Tips for searching another platform that hosts library databases

There is no magic number regarding how many sources you are going to need for your literature review; it all depends on the topic and what type of the literature review you are doing:

► Are you working on an emerging topic? You are not likely to find many sources, which is good because you are trying to prove that this is a topic that needs more research. But, it is not enough to say that you found few or no articles on your topic in your field. You need to look broadly to other disciplines (also known as triangulation ) to see if your research topic has been studied from other perspectives as a way to validate the uniqueness of your research question.

► Are you working on something that has been studied extensively? Then you are going to find many sources and you will want to limit how far back you want to look. Use limiters to eliminate research that may be dated and opt to search for resources published within the last 5-10 years.

• << Previous: How to Pick a Topic
• Next: Evaluating Sources & Lit. Reviews >>
• Last Updated: Sep 21, 2022 2:16 PM
• URL: https://guides.lib.uconn.edu/literaturereview

• Chester Fritz Library
• Library of the Health Sciences
• Thormodsgard Law Library

Literature Reviews

• Get started
• What is a Literature Review?
• Finding Literature Reviews

Literature Review in a Paper

Systematic/semi-systematic literature reviews, defining your research question, where will you search, how will you search, documenting & organizing, types of literature.

• Library Books
• How to Videos
• Communicating & Citing Research
• Bibliography

On This Page:

• Your Search Strategy
• Search Strategy Resources
• Search Techniques
• Documenting & Organizing Your Search
• References & Further Reading

Since you are situating your research within the larger scholarly conversation rather than summarizing everything that has been written about a topic, here are some questions that can help you decide what to include in your literature review:

• Authors who wrote about your topic or a similar topic
• Most frequently cited works/authors on your topic
• Who identified the research gap that you seek to address?
• What conflicts are there about your topic?
• What has been most recently written about your topic?

Strategies for identifying important authors on a topic include:

• Looking up encyclopedia entries on your topic. Encylopedias generally reach out to topic experts, inviting them to write the relevant chapter. Thus, both the entry authors and the citations they list are great starting points for your research.
• Look up your topic in a citation tracking database such as Web of Science and see which articles and authors are cited the most on your topic. Also take a look at who has cited those articles/authors to make sure they are citing them for positive reasons.

Both systematic and semi-systematic literature reviews require you to establish a search methodology before conducting your literature review. You will need to identify:

• Where you will search (which databases, journals, other resources)
• How you will search (search terms, search strategies)
• What you will include/exclude from your results (specific criteria)

You will also want to take detailed notes as you search. See Library Books and Documenting and Organizing for resources to help guide you in your search.

It is integral to spend time honing and defining your research question before searching the literature. Here are a couple tools used for by particular science and social science disciplines to help you define your research question:

• PICOT  / PICO  (quantitative evidence-based research/synthesis) and
• SPIDER  (qualitative evidence-based research/synthesis) 

PICO (Quantitative) and SPIDER (Qualitative)

Cooke, Smith, & Booth (2012).

*The "T" (PICOT) is left out of the above study. It represents Time, or the duration of data collection (Riva, Malik, Burnie, Endicott, & Busse, 2012)

Engineering PICO*

P = Population, Problem, Process

I = Intervention, Inquiry, Investigation, Improvement

C = Comparison (current practice or opposing viewpoints)

O = Outcomes (measuring what worked best)

*Read more about it on Arizona State University Library's "Engineering -- Formulating questions w/PICO" guide:  https://libguides.asu.edu/engineering/PICO

Your research topic and type of literature review will help you determine where to look.

For literature reviews within a paper, you will likely at least want to search an important subject database and a citation tracking database.

• Subject Research Guides can help you identity important subject databases
• Web of Science and Google Scholar are citation tracking databases

For systematic/semi-systematic literature reviews, you will likely be more comprehensive in your search. In addition to the databases mentioned above, you may want to:

• Use the Library Search and "Expand Beyond Library" to search everything indexed by UND library databases and additional sources, such as open access materials
• Search WorldCat or/and Google Books, particularly for humanities disciplines
• Search government documents or other gray literature resources relevant for your discipline

Dissertations and Theses can also help you with a literature review, as these tend to include thorough literature reviews on a topic. Take a look at their literature review section and citations.

• CFL Research Guides Identifies research starting points for different subjects.

Dissertations from 1861 - present. Master's theses from 1988 - present. Includes full text from both UND and external dissertations and theses. Hosted by Proquest.

Search Strategies

Conventional subject searching in databases.

Subject database searching generally includes developing a search strategy around subject terms, reflecting aspects of the research question. You may want to use Booleans (AND/OR/NOT) and wild card operators (*/!) to help you create a thorough and precise search strategy. Searches are often restricted by language and date, and sometimes geographic region, through the use of database  limiters .

Example research question and search strategy

Research question: Is there a correlation between fast food advertising and childhood obesity?

Prelude to developing a search strategy: How could that correlation be shown? Perhaps the number of ads by fast food companies over time and childhood obesity over time? How can I tell whether those ads target children? Perhaps if the ads include cartoons or toys or character mascots they can be considered to target children; perhaps previous research will help me identify additional methods, as well. What words could be used to describe "fast food," "advertising," "children" and "obesity"?

Initial search strategy: (kid* or child*) AND (market* OR advertis*) AND "fast food" AND (obesity OR weight OR fat)

Updated search strategy after initial search: (kid* or child*) AND (market* OR advertis*) AND ("fast food" OR "quick service")

Citation Chaining & Citation Searching (Backward & Forward Snowballing)

These techniques refer to checking reference lists and citing articles (articles that have cited the article that you are currently looking at). Citation chaining involves checking references on all included papers identified by various search methods so that relevant references not yet identified can be added to the pool of included studies. It also includes checking articles that cited an included paper. Many research databases link citing articles to each article record. Databases that are useful for citation searching include Google Scholar, Web of Science, CINAHL Complete, Wiley Online, and others. Access Chester Fritz Library's most used databases by visiting our  home page   and clicking on QuickLinks or the complete list by visiting our  A-Z Databases page .

Traditional vs. Comprehensive Pearl Growing

Traditional Pearl Growing (TPG) begins with one or more target articles, judged to be such due to their relevancy to the research topic. The target article is called a pearl. It's a step beyond the citation chaining and searching methods. The researcher then identifies keywords to add to their search from aspects of the article (e.g., abstract, subject terms, author, etc.). Hawkins and Wagers (1982) coined this process as "growing more pearls" (as cited in  Schlosser,  Wendt ,  Bhavnani , & Nail‐Chiwetalu, 2006).

Comprehensive Pearl Growing (CPG) involves the following process: (1) Start with a compilation of studies from a relevant review or a topical bibliography; (2) determine relevant databases for these studies; (3) determine how these studies are indexed in database 1 in terms of keywords and quality filters; (4) find other relevant articles in database 1 (or as many are relevant) using the index terms in a Building Block query; and (5) end when articles retrieved provide diminishing relevance. Thus, rather than beginning with only one pearl, CPG requires of the searcher to begin with a compilation of studies from a relevant narrative review or a topical bibliography. Like TPG, CPG makes use of existing studies to determine the keywords and quality filters under which they are indexed in order to retrieve more articles of the same kind   (Schlosser, Wendt, Bhavnani, & Nail‐Chiwetalu, 2006).

Although pearl growing techniques are effective across disciplines, they may be particularly strategic for interdisciplinary research questions in which multiple controlled vocabularies (e.g., thesauri, database subject terms, discipline-specific terminology), are integral to pulling together  sources across research databases (Schlosser, Wendt, Bhavnani , & Nail‐Chiwetalu, 2006).

Text-Mining

In Software Engineering, various text-mining (TM) techniques are used more and more to implement systematic literature review processes, however further research is needed--read Feng, Chiam, and Lo (2017) linked below for more information.

Document Your Literature Search

Use paper and pen, the below excel file, or online tools or applications like Trello to set up a system for documenting your search strategy. This contributes to research transparency and gives you a mechanism to provide quick and accurate documentation of your search strategies when pre-registering systematic review protocol or being questioned about how you searched the literature (and what you may have missed) by supervisors, colleagues, or reviewers.

• Search Strategy Documentation Template Be systematic by documenting your search strategy (keywords, databases, etc.). This helps you to remember what you have done before and provides documentation for research transparency.

For systematic reviews or meta-analyses, use the PRISMA or MOOSE checklists to evaluate each included resource for inclusion.

• PRISMA Checklist Transparent Reporting of Systematic Reviews and Meta-analyses.
• MOOSE Guidelines for Meta-Analyses and Systematic Reviews of Observational Studies* *Modified from Stroup DF, Berlin JA, Morton SC, Olkin I, Williamson GD, Rennie D, et al. Meta-analysis Of Observational Studies in Epidemiology (MOOSE) group. JAMA 2000;283:2008–12.

Organize Your Literature

• Citation Managers A research guide providing information and resources for a handful of the most popular citation managers.
• EndNote @ UND Page of the Citation Managers research guide discussing UND's EndNote subscription and use.

• Mendeley Mendeley (Elsevier) is a free reference manager and an academic social network. Manage your research, showcase your work, connect and collaborate.
• Zotero Zotero is a free, easy-to-use citation management tool to help you collect, organize, cite, and share research.

Where Will You Look?

The literature you gather greatly depends upon the sources that you look in. Studies appearing in peer-reviewed journals are easy to locate but will likely over-represent significant and novel results, while certain types of grey literature (e.g., dissertations and theses; self-published manuscripts; unpublished studies; conference abstracts, presentations, and proceedings; regulatory data; unpublished trial data; government publications; reports such as white papers, working papers, and internal documentation; patents; and policies & procedures) may be more difficult to find and access in full text--for example, you may need to contact authors or organizations directly. It is good practice to use listserv and distribution lists for this type of material along with direct personal contacts, keeping in mind that the latter may bias the results towards those in support of a particular contact's central beliefs and research results (Cooper, 2010).

Obviously, this means that limiting your search to journals in databases may skew results towards statistically significant findings, biasing your pool of studies which would be lacking in null, or inconclusive, results. You can also search for grey literature in institutional repositories like UND Scholarly Commons , government/professional organizations and conference websites, Open Data Repositories , open preprint repositories, theses and dissertation databases, online Researcher Communities , and journals that publish Registered Reports  or null and inconclusive findings like PLOS ONE .

Author's Versions & Grey Literature Database Examples:

• Open Science Framework Search Search OSF projects and data files. OSF is a free and open source project management repository that supports researchers across their entire project lifecycle.

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• Last Updated: Dec 5, 2023 8:31 PM
• URL: https://libguides.und.edu/literature-reviews

Literature Searching

In this guide.

• Introduction
• Steps for searching the literature in PubMed
• Step 1 - Formulate a search question
• Step 2- Identify primary concepts and gather synonyms
• Step 3 - Locate subject headings (MeSH)
• Step 4 - Combine concepts using Boolean operators
• Step 5 - Refine search terms and search in PubMed
• Step 6 - Apply limits

Students and researchers in the health sciences are often required to conduct literature searches for a number of reasons including identifying appropriate studies and methods to include in a literature review manuscript. Understanding the basics of database searching can allow you to effectively and efficiently find the information you need. This guide takes you through the process of developing an advanced, robust literature search in PubMed . 

While the guide is based on searching in the PubMed database, the strategies can be applied appropriately to other databases, such as Embase , CINAHL , PsycINFO , etc. (see search syntax for more information on search translation). For more information on searching in other databases, attend Lane's Literature Reviews Beyond PubMed: Crafting Effective Searches in Other Databases  course.

If you're interested in conducting a systematic review, please visit Introduction to Systematic Reviews . 

Research Services

• Literature Searches
• Data Service
• Research Metrics Service
• Authoring and Publishing Support

Literature or reviews of a systematic nature is a value-added service available for current members of the Stanford Medicine community. To qualify for the service, the project lead and point-of-contact to Lane must be a Stanford affiliate.

Lane Medical Library staff can help you with your search strategy creation, collaborate on systematic reviews and other knowledge syntheses, provide guidance on documentation, processes, and tools, among others. Reviews or projects the team can help you with include but are not limited to:

• Systematic Review
• Meta-Analysis
• Literature Review for Grant Application
• Thesis or Dissertation
• Course Assignment
• Scoping Review
• Book Chapter
• Thesis Report

If you are interested in collaborating with a Lane Medical Librarian on a review project, please submit a  literature search request .

Data management and sharing is a component of Open Science, which aims to make scientific research more transparent and accessible. Proper data management and sharing benefit you as an individual and the research community as a whole. Lane's data service provides: 

• Best practices related to data management and sharing
• Assistance in complying with requirements related to the management and sharing of research data (e.g. from a publisher or funder)
• Consultations related to research data management, data security, data publishing, data curation, and long-term preservation
• Workshops and classes related to best practices in data management and sharing

For more information, visit the Data Management and Sharing guide  or contact data librarian John Borghi ([email protected]). 

Research metrics measure the impact of a scholar, article, book, journal, or research institution. Metrics can be informed through different approaches, such as citation counts, that can add to a broader understanding of impact. Lane's research metrics service assists with:

• Understanding, measuring and using bibliometrics and other statistical analysis of publications 
• Using alternative metrics (also known as altmetrics) 
• Common tools for assessing research impact

For more information on research metrics, visit the Research Impact guide or contact Research Communications Librarian Lily Ren ([email protected]).

Lane Librarians can help you with the authoring and publishing of your manuscript such as:

• Learn about unique author identifiers and how to create an ORCID ID
• How to find journals relevant to areas of research/interest for possible submission
• How to find journal impact factors
• Help find instructions for authors
• Explore alternate publishing models including open access journals
• How to verify citations

We also provide discounted Open Access author processing fees. For more information, visit our Understanding Open Access guide.  

• Next: Steps for searching the literature in PubMed >>
• Last Updated: Jan 9, 2024 10:30 AM
• URL: https://laneguides.stanford.edu/LitSearch

Literature Review: Developing a search strategy

• Traditional or narrative literature reviews
• Scoping Reviews
• Systematic literature reviews
• Annotated bibliography
• Keeping up to date with literature
• Finding a thesis
• Evaluating sources and critical appraisal of literature
• Managing and analysing your literature
• Further reading and resources

From research question to search strategy

Keeping a record of your search activity

Good search practice could involve keeping a search diary or document detailing your search activities (Phelps et. al. 2007, pp. 128-149), so that you can keep track of effective search terms, or to help others to reproduce your steps and get the same results. 

This record could be a document, table or spreadsheet with:

• The names of the sources you search and which provider you accessed them through - eg Medline (Ovid), Web of Science (Thomson Reuters). You should also include any other literature sources you used.
• how you searched (keyword and/or subject headings)
• which search terms you used (which words and phrases)
• any search techniques you employed (truncation, adjacency, etc)
• how you combined your search terms (AND/OR). Check out the Database Help guide for more tips on Boolean Searching.
• The number of search results from each source and each strategy used. This can be the evidence you need to prove a gap in the literature, and confirms the importance of your research question.

A search planner may help you to organise you thoughts prior to conducting your search. If you have any problems with organising your thoughts prior, during and after searching please contact your Library  Faculty Team   for individual help.

• Literature search - a librarian's handout to introduce tools, terms and techniques Created by Elsevier librarian, Katy Kavanagh Web, this document outlines tools, terms and techniques to think about when conducting a literature search.
• Search planner

Literature search cycle

Diagram text description

This diagram illustrates the literature search cycle. It shows a circle in quarters. Top left quarter is identify main concepts with rectangle describing how to do this by identifying:controlled vocabulary terms, synonyms, keywords and spelling. Top right quarter select library resources to search and rectangle describing resources to search library catalogue relevant journal articles and other resource. Bottom right corner of circle search resources and in rectangle consider using boolean searching proximity searching and truncated searching techniques. Bottom left quarter of circle review and refine results. In rectangle evaluate results, rethink keywords and create alerts.

Have a search framework

Search frameworks are mnemonics which can help you focus your research question. They are also useful in helping you to identify the concepts and terms you will use in your literature search.

PICO is a search framework commonly used in the health sciences to focus clinical questions.  As an example, you work in an aged care facility and are interested in whether cranberry juice might help reduce the common occurrence of urinary tract infections.  The PICO framework would look like this:

Now that the issue has been broken up to its elements, it is easier to turn it into an answerable research question: “Does cranberry juice help reduce urinary tract infections in people living in aged care facilities?”

Other frameworks may be helpful, depending on your question and your field of interest. PICO can be adapted to PICOT (which adds T ime) or PICOS (which adds S tudy design), or PICOC (adding C ontext).

For qualitative questions you could use

• SPIDER : S ample,  P henomenon of  I nterest,  D esign,  E valuation,  R esearch type  

For questions about causes or risk,

• PEO : P opulation,  E xposure,  O utcomes

For evaluations of interventions or policies, 

• SPICE: S etting,  P opulation or  P erspective,  I ntervention,  C omparison,  E valuation or
• ECLIPSE: E xpectation,  C lient group,  L ocation,  I mpact,  P rofessionals,  SE rvice 

See the University of Notre Dame Australia’s examples of some of these frameworks. 

You can also try some PICO examples in the National Library of Medicine's PubMed training site: Using PICO to frame clinical questions.

Contact Your Faculty Team Librarian

Faculty librarians are here to provide assistance to students, researchers and academic staff by providing expert searching advice, research and curriculum support.

• Faculty of Arts & Education team
• Faculty of Business, Justice & Behavioural Science team
• Faculty of Science team

Further reading

• << Previous: Annotated bibliography
• Next: Keeping up to date with literature >>
• Last Updated: Apr 10, 2024 5:05 PM
• URL: https://libguides.csu.edu.au/review

Charles Sturt University is an Australian University, TEQSA Provider Identification: PRV12018. CRICOS Provider: 00005F.

• Open access
• Published: 14 August 2018

Defining the process to literature searching in systematic reviews: a literature review of guidance and supporting studies

• Chris Cooper   ORCID: orcid.org/0000-0003-0864-5607 1 ,
• Andrew Booth 2 ,
• Jo Varley-Campbell 1 ,
• Nicky Britten 3 &
• Ruth Garside 4  

BMC Medical Research Methodology volume  18 , Article number:  85 ( 2018 ) Cite this article

200k Accesses

203 Citations

118 Altmetric

Metrics details

Systematic literature searching is recognised as a critical component of the systematic review process. It involves a systematic search for studies and aims for a transparent report of study identification, leaving readers clear about what was done to identify studies, and how the findings of the review are situated in the relevant evidence.

Information specialists and review teams appear to work from a shared and tacit model of the literature search process. How this tacit model has developed and evolved is unclear, and it has not been explicitly examined before.

The purpose of this review is to determine if a shared model of the literature searching process can be detected across systematic review guidance documents and, if so, how this process is reported in the guidance and supported by published studies.

A literature review.

Two types of literature were reviewed: guidance and published studies. Nine guidance documents were identified, including: The Cochrane and Campbell Handbooks. Published studies were identified through ‘pearl growing’, citation chasing, a search of PubMed using the systematic review methods filter, and the authors’ topic knowledge.

The relevant sections within each guidance document were then read and re-read, with the aim of determining key methodological stages. Methodological stages were identified and defined. This data was reviewed to identify agreements and areas of unique guidance between guidance documents. Consensus across multiple guidance documents was used to inform selection of ‘key stages’ in the process of literature searching.

Eight key stages were determined relating specifically to literature searching in systematic reviews. They were: who should literature search, aims and purpose of literature searching, preparation, the search strategy, searching databases, supplementary searching, managing references and reporting the search process.

Conclusions

Eight key stages to the process of literature searching in systematic reviews were identified. These key stages are consistently reported in the nine guidance documents, suggesting consensus on the key stages of literature searching, and therefore the process of literature searching as a whole, in systematic reviews. Further research to determine the suitability of using the same process of literature searching for all types of systematic review is indicated.

Peer Review reports

Systematic literature searching is recognised as a critical component of the systematic review process. It involves a systematic search for studies and aims for a transparent report of study identification, leaving review stakeholders clear about what was done to identify studies, and how the findings of the review are situated in the relevant evidence.

Information specialists and review teams appear to work from a shared and tacit model of the literature search process. How this tacit model has developed and evolved is unclear, and it has not been explicitly examined before. This is in contrast to the information science literature, which has developed information processing models as an explicit basis for dialogue and empirical testing. Without an explicit model, research in the process of systematic literature searching will remain immature and potentially uneven, and the development of shared information models will be assumed but never articulated.

One way of developing such a conceptual model is by formally examining the implicit “programme theory” as embodied in key methodological texts. The aim of this review is therefore to determine if a shared model of the literature searching process in systematic reviews can be detected across guidance documents and, if so, how this process is reported and supported.

Identifying guidance

Key texts (henceforth referred to as “guidance”) were identified based upon their accessibility to, and prominence within, United Kingdom systematic reviewing practice. The United Kingdom occupies a prominent position in the science of health information retrieval, as quantified by such objective measures as the authorship of papers, the number of Cochrane groups based in the UK, membership and leadership of groups such as the Cochrane Information Retrieval Methods Group, the HTA-I Information Specialists’ Group and historic association with such centres as the UK Cochrane Centre, the NHS Centre for Reviews and Dissemination, the Centre for Evidence Based Medicine and the National Institute for Clinical Excellence (NICE). Coupled with the linguistic dominance of English within medical and health science and the science of systematic reviews more generally, this offers a justification for a purposive sample that favours UK, European and Australian guidance documents.

Nine guidance documents were identified. These documents provide guidance for different types of reviews, namely: reviews of interventions, reviews of health technologies, reviews of qualitative research studies, reviews of social science topics, and reviews to inform guidance.

Whilst these guidance documents occasionally offer additional guidance on other types of systematic reviews, we have focused on the core and stated aims of these documents as they relate to literature searching. Table  1 sets out: the guidance document, the version audited, their core stated focus, and a bibliographical pointer to the main guidance relating to literature searching.

Once a list of key guidance documents was determined, it was checked by six senior information professionals based in the UK for relevance to current literature searching in systematic reviews.

Identifying supporting studies

In addition to identifying guidance, the authors sought to populate an evidence base of supporting studies (henceforth referred to as “studies”) that contribute to existing search practice. Studies were first identified by the authors from their knowledge on this topic area and, subsequently, through systematic citation chasing key studies (‘pearls’ [ 1 ]) located within each key stage of the search process. These studies are identified in Additional file  1 : Appendix Table 1. Citation chasing was conducted by analysing the bibliography of references for each study (backwards citation chasing) and through Google Scholar (forward citation chasing). A search of PubMed using the systematic review methods filter was undertaken in August 2017 (see Additional file 1 ). The search terms used were: (literature search*[Title/Abstract]) AND sysrev_methods[sb] and 586 results were returned. These results were sifted for relevance to the key stages in Fig.  1 by CC.

The key stages of literature search guidance as identified from nine key texts

Extracting the data

To reveal the implicit process of literature searching within each guidance document, the relevant sections (chapters) on literature searching were read and re-read, with the aim of determining key methodological stages. We defined a key methodological stage as a distinct step in the overall process for which specific guidance is reported, and action is taken, that collectively would result in a completed literature search.

The chapter or section sub-heading for each methodological stage was extracted into a table using the exact language as reported in each guidance document. The lead author (CC) then read and re-read these data, and the paragraphs of the document to which the headings referred, summarising section details. This table was then reviewed, using comparison and contrast to identify agreements and areas of unique guidance. Consensus across multiple guidelines was used to inform selection of ‘key stages’ in the process of literature searching.

Having determined the key stages to literature searching, we then read and re-read the sections relating to literature searching again, extracting specific detail relating to the methodological process of literature searching within each key stage. Again, the guidance was then read and re-read, first on a document-by-document-basis and, secondly, across all the documents above, to identify both commonalities and areas of unique guidance.

Results and discussion

Our findings.

We were able to identify consensus across the guidance on literature searching for systematic reviews suggesting a shared implicit model within the information retrieval community. Whilst the structure of the guidance varies between documents, the same key stages are reported, even where the core focus of each document is different. We were able to identify specific areas of unique guidance, where a document reported guidance not summarised in other documents, together with areas of consensus across guidance.

Unique guidance

Only one document provided guidance on the topic of when to stop searching [ 2 ]. This guidance from 2005 anticipates a topic of increasing importance with the current interest in time-limited (i.e. “rapid”) reviews. Quality assurance (or peer review) of literature searches was only covered in two guidance documents [ 3 , 4 ]. This topic has emerged as increasingly important as indicated by the development of the PRESS instrument [ 5 ]. Text mining was discussed in four guidance documents [ 4 , 6 , 7 , 8 ] where the automation of some manual review work may offer efficiencies in literature searching [ 8 ].

Agreement between guidance: Defining the key stages of literature searching

Where there was agreement on the process, we determined that this constituted a key stage in the process of literature searching to inform systematic reviews.

From the guidance, we determined eight key stages that relate specifically to literature searching in systematic reviews. These are summarised at Fig. 1 . The data extraction table to inform Fig. 1 is reported in Table  2 . Table 2 reports the areas of common agreement and it demonstrates that the language used to describe key stages and processes varies significantly between guidance documents.

For each key stage, we set out the specific guidance, followed by discussion on how this guidance is situated within the wider literature.

Key stage one: Deciding who should undertake the literature search

The guidance.

Eight documents provided guidance on who should undertake literature searching in systematic reviews [ 2 , 4 , 6 , 7 , 8 , 9 , 10 , 11 ]. The guidance affirms that people with relevant expertise of literature searching should ‘ideally’ be included within the review team [ 6 ]. Information specialists (or information scientists), librarians or trial search co-ordinators (TSCs) are indicated as appropriate researchers in six guidance documents [ 2 , 7 , 8 , 9 , 10 , 11 ].

How the guidance corresponds to the published studies

The guidance is consistent with studies that call for the involvement of information specialists and librarians in systematic reviews [ 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 ] and which demonstrate how their training as ‘expert searchers’ and ‘analysers and organisers of data’ can be put to good use [ 13 ] in a variety of roles [ 12 , 16 , 20 , 21 , 24 , 25 , 26 ]. These arguments make sense in the context of the aims and purposes of literature searching in systematic reviews, explored below. The need for ‘thorough’ and ‘replicable’ literature searches was fundamental to the guidance and recurs in key stage two. Studies have found poor reporting, and a lack of replicable literature searches, to be a weakness in systematic reviews [ 17 , 18 , 27 , 28 ] and they argue that involvement of information specialists/ librarians would be associated with better reporting and better quality literature searching. Indeed, Meert et al. [ 29 ] demonstrated that involving a librarian as a co-author to a systematic review correlated with a higher score in the literature searching component of a systematic review [ 29 ]. As ‘new styles’ of rapid and scoping reviews emerge, where decisions on how to search are more iterative and creative, a clear role is made here too [ 30 ].

Knowing where to search for studies was noted as important in the guidance, with no agreement as to the appropriate number of databases to be searched [ 2 , 6 ]. Database (and resource selection more broadly) is acknowledged as a relevant key skill of information specialists and librarians [ 12 , 15 , 16 , 31 ].

Whilst arguments for including information specialists and librarians in the process of systematic review might be considered self-evident, Koffel and Rethlefsen [ 31 ] have questioned if the necessary involvement is actually happening [ 31 ].

Key stage two: Determining the aim and purpose of a literature search

The aim: Five of the nine guidance documents use adjectives such as ‘thorough’, ‘comprehensive’, ‘transparent’ and ‘reproducible’ to define the aim of literature searching [ 6 , 7 , 8 , 9 , 10 ]. Analogous phrases were present in a further three guidance documents, namely: ‘to identify the best available evidence’ [ 4 ] or ‘the aim of the literature search is not to retrieve everything. It is to retrieve everything of relevance’ [ 2 ] or ‘A systematic literature search aims to identify all publications relevant to the particular research question’ [ 3 ]. The Joanna Briggs Institute reviewers’ manual was the only guidance document where a clear statement on the aim of literature searching could not be identified. The purpose of literature searching was defined in three guidance documents, namely to minimise bias in the resultant review [ 6 , 8 , 10 ]. Accordingly, eight of nine documents clearly asserted that thorough and comprehensive literature searches are required as a potential mechanism for minimising bias.

The need for thorough and comprehensive literature searches appears as uniform within the eight guidance documents that describe approaches to literature searching in systematic reviews of effectiveness. Reviews of effectiveness (of intervention or cost), accuracy and prognosis, require thorough and comprehensive literature searches to transparently produce a reliable estimate of intervention effect. The belief that all relevant studies have been ‘comprehensively’ identified, and that this process has been ‘transparently’ reported, increases confidence in the estimate of effect and the conclusions that can be drawn [ 32 ]. The supporting literature exploring the need for comprehensive literature searches focuses almost exclusively on reviews of intervention effectiveness and meta-analysis. Different ‘styles’ of review may have different standards however; the alternative, offered by purposive sampling, has been suggested in the specific context of qualitative evidence syntheses [ 33 ].

What is a comprehensive literature search?

Whilst the guidance calls for thorough and comprehensive literature searches, it lacks clarity on what constitutes a thorough and comprehensive literature search, beyond the implication that all of the literature search methods in Table 2 should be used to identify studies. Egger et al. [ 34 ], in an empirical study evaluating the importance of comprehensive literature searches for trials in systematic reviews, defined a comprehensive search for trials as:

a search not restricted to English language;

where Cochrane CENTRAL or at least two other electronic databases had been searched (such as MEDLINE or EMBASE); and

at least one of the following search methods has been used to identify unpublished trials: searches for (I) conference abstracts, (ii) theses, (iii) trials registers; and (iv) contacts with experts in the field [ 34 ].

Tricco et al. (2008) used a similar threshold of bibliographic database searching AND a supplementary search method in a review when examining the risk of bias in systematic reviews. Their criteria were: one database (limited using the Cochrane Highly Sensitive Search Strategy (HSSS)) and handsearching [ 35 ].

Together with the guidance, this would suggest that comprehensive literature searching requires the use of BOTH bibliographic database searching AND supplementary search methods.

Comprehensiveness in literature searching, in the sense of how much searching should be undertaken, remains unclear. Egger et al. recommend that ‘investigators should consider the type of literature search and degree of comprehension that is appropriate for the review in question, taking into account budget and time constraints’ [ 34 ]. This view tallies with the Cochrane Handbook, which stipulates clearly, that study identification should be undertaken ‘within resource limits’ [ 9 ]. This would suggest that the limitations to comprehension are recognised but it raises questions on how this is decided and reported [ 36 ].

What is the point of comprehensive literature searching?

The purpose of thorough and comprehensive literature searches is to avoid missing key studies and to minimize bias [ 6 , 8 , 10 , 34 , 37 , 38 , 39 ] since a systematic review based only on published (or easily accessible) studies may have an exaggerated effect size [ 35 ]. Felson (1992) sets out potential biases that could affect the estimate of effect in a meta-analysis [ 40 ] and Tricco et al. summarize the evidence concerning bias and confounding in systematic reviews [ 35 ]. Egger et al. point to non-publication of studies, publication bias, language bias and MEDLINE bias, as key biases [ 34 , 35 , 40 , 41 , 42 , 43 , 44 , 45 , 46 ]. Comprehensive searches are not the sole factor to mitigate these biases but their contribution is thought to be significant [ 2 , 32 , 34 ]. Fehrmann (2011) suggests that ‘the search process being described in detail’ and that, where standard comprehensive search techniques have been applied, increases confidence in the search results [ 32 ].

Does comprehensive literature searching work?

Egger et al., and other study authors, have demonstrated a change in the estimate of intervention effectiveness where relevant studies were excluded from meta-analysis [ 34 , 47 ]. This would suggest that missing studies in literature searching alters the reliability of effectiveness estimates. This is an argument for comprehensive literature searching. Conversely, Egger et al. found that ‘comprehensive’ searches still missed studies and that comprehensive searches could, in fact, introduce bias into a review rather than preventing it, through the identification of low quality studies then being included in the meta-analysis [ 34 ]. Studies query if identifying and including low quality or grey literature studies changes the estimate of effect [ 43 , 48 ] and question if time is better invested updating systematic reviews rather than searching for unpublished studies [ 49 ], or mapping studies for review as opposed to aiming for high sensitivity in literature searching [ 50 ].

Aim and purpose beyond reviews of effectiveness

The need for comprehensive literature searches is less certain in reviews of qualitative studies, and for reviews where a comprehensive identification of studies is difficult to achieve (for example, in Public health) [ 33 , 51 , 52 , 53 , 54 , 55 ]. Literature searching for qualitative studies, and in public health topics, typically generates a greater number of studies to sift than in reviews of effectiveness [ 39 ] and demonstrating the ‘value’ of studies identified or missed is harder [ 56 ], since the study data do not typically support meta-analysis. Nussbaumer-Streit et al. (2016) have registered a review protocol to assess whether abbreviated literature searches (as opposed to comprehensive literature searches) has an impact on conclusions across multiple bodies of evidence, not only on effect estimates [ 57 ] which may develop this understanding. It may be that decision makers and users of systematic reviews are willing to trade the certainty from a comprehensive literature search and systematic review in exchange for different approaches to evidence synthesis [ 58 ], and that comprehensive literature searches are not necessarily a marker of literature search quality, as previously thought [ 36 ]. Different approaches to literature searching [ 37 , 38 , 59 , 60 , 61 , 62 ] and developing the concept of when to stop searching are important areas for further study [ 36 , 59 ].

The study by Nussbaumer-Streit et al. has been published since the submission of this literature review [ 63 ]. Nussbaumer-Streit et al. (2018) conclude that abbreviated literature searches are viable options for rapid evidence syntheses, if decision-makers are willing to trade the certainty from a comprehensive literature search and systematic review, but that decision-making which demands detailed scrutiny should still be based on comprehensive literature searches [ 63 ].

Key stage three: Preparing for the literature search

Six documents provided guidance on preparing for a literature search [ 2 , 3 , 6 , 7 , 9 , 10 ]. The Cochrane Handbook clearly stated that Cochrane authors (i.e. researchers) should seek advice from a trial search co-ordinator (i.e. a person with specific skills in literature searching) ‘before’ starting a literature search [ 9 ].

Two key tasks were perceptible in preparing for a literature searching [ 2 , 6 , 7 , 10 , 11 ]. First, to determine if there are any existing or on-going reviews, or if a new review is justified [ 6 , 11 ]; and, secondly, to develop an initial literature search strategy to estimate the volume of relevant literature (and quality of a small sample of relevant studies [ 10 ]) and indicate the resources required for literature searching and the review of the studies that follows [ 7 , 10 ].

Three documents summarised guidance on where to search to determine if a new review was justified [ 2 , 6 , 11 ]. These focused on searching databases of systematic reviews (The Cochrane Database of Systematic Reviews (CDSR) and the Database of Abstracts of Reviews of Effects (DARE)), institutional registries (including PROSPERO), and MEDLINE [ 6 , 11 ]. It is worth noting, however, that as of 2015, DARE (and NHS EEDs) are no longer being updated and so the relevance of this (these) resource(s) will diminish over-time [ 64 ]. One guidance document, ‘Systematic reviews in the Social Sciences’, noted, however, that databases are not the only source of information and unpublished reports, conference proceeding and grey literature may also be required, depending on the nature of the review question [ 2 ].

Two documents reported clearly that this preparation (or ‘scoping’) exercise should be undertaken before the actual search strategy is developed [ 7 , 10 ]).

The guidance offers the best available source on preparing the literature search with the published studies not typically reporting how their scoping informed the development of their search strategies nor how their search approaches were developed. Text mining has been proposed as a technique to develop search strategies in the scoping stages of a review although this work is still exploratory [ 65 ]. ‘Clustering documents’ and word frequency analysis have also been tested to identify search terms and studies for review [ 66 , 67 ]. Preparing for literature searches and scoping constitutes an area for future research.

Key stage four: Designing the search strategy

The Population, Intervention, Comparator, Outcome (PICO) structure was the commonly reported structure promoted to design a literature search strategy. Five documents suggested that the eligibility criteria or review question will determine which concepts of PICO will be populated to develop the search strategy [ 1 , 4 , 7 , 8 , 9 ]. The NICE handbook promoted multiple structures, namely PICO, SPICE (Setting, Perspective, Intervention, Comparison, Evaluation) and multi-stranded approaches [ 4 ].

With the exclusion of The Joanna Briggs Institute reviewers’ manual, the guidance offered detail on selecting key search terms, synonyms, Boolean language, selecting database indexing terms and combining search terms. The CEE handbook suggested that ‘search terms may be compiled with the help of the commissioning organisation and stakeholders’ [ 10 ].

The use of limits, such as language or date limits, were discussed in all documents [ 2 , 3 , 4 , 6 , 7 , 8 , 9 , 10 , 11 ].

Search strategy structure

The guidance typically relates to reviews of intervention effectiveness so PICO – with its focus on intervention and comparator - is the dominant model used to structure literature search strategies [ 68 ]. PICOs – where the S denotes study design - is also commonly used in effectiveness reviews [ 6 , 68 ]. As the NICE handbook notes, alternative models to structure literature search strategies have been developed and tested. Booth provides an overview on formulating questions for evidence based practice [ 69 ] and has developed a number of alternatives to the PICO structure, namely: BeHEMoTh (Behaviour of interest; Health context; Exclusions; Models or Theories) for use when systematically identifying theory [ 55 ]; SPICE (Setting, Perspective, Intervention, Comparison, Evaluation) for identification of social science and evaluation studies [ 69 ] and, working with Cooke and colleagues, SPIDER (Sample, Phenomenon of Interest, Design, Evaluation, Research type) [ 70 ]. SPIDER has been compared to PICO and PICOs in a study by Methley et al. [ 68 ].

The NICE handbook also suggests the use of multi-stranded approaches to developing literature search strategies [ 4 ]. Glanville developed this idea in a study by Whitting et al. [ 71 ] and a worked example of this approach is included in the development of a search filter by Cooper et al. [ 72 ].

Writing search strategies: Conceptual and objective approaches

Hausner et al. [ 73 ] provide guidance on writing literature search strategies, delineating between conceptually and objectively derived approaches. The conceptual approach, advocated by and explained in the guidance documents, relies on the expertise of the literature searcher to identify key search terms and then develop key terms to include synonyms and controlled syntax. Hausner and colleagues set out the objective approach [ 73 ] and describe what may be done to validate it [ 74 ].

The use of limits

The guidance documents offer direction on the use of limits within a literature search. Limits can be used to focus literature searching to specific study designs or by other markers (such as by date) which limits the number of studies returned by a literature search. The use of limits should be described and the implications explored [ 34 ] since limiting literature searching can introduce bias (explored above). Craven et al. have suggested the use of a supporting narrative to explain decisions made in the process of developing literature searches and this advice would usefully capture decisions on the use of search limits [ 75 ].

Key stage five: Determining the process of literature searching and deciding where to search (bibliographic database searching)

Table 2 summarises the process of literature searching as reported in each guidance document. Searching bibliographic databases was consistently reported as the ‘first step’ to literature searching in all nine guidance documents.

Three documents reported specific guidance on where to search, in each case specific to the type of review their guidance informed, and as a minimum requirement [ 4 , 9 , 11 ]. Seven of the key guidance documents suggest that the selection of bibliographic databases depends on the topic of review [ 2 , 3 , 4 , 6 , 7 , 8 , 10 ], with two documents noting the absence of an agreed standard on what constitutes an acceptable number of databases searched [ 2 , 6 ].

The guidance documents summarise ‘how to’ search bibliographic databases in detail and this guidance is further contextualised above in terms of developing the search strategy. The documents provide guidance of selecting bibliographic databases, in some cases stating acceptable minima (i.e. The Cochrane Handbook states Cochrane CENTRAL, MEDLINE and EMBASE), and in other cases simply listing bibliographic database available to search. Studies have explored the value in searching specific bibliographic databases, with Wright et al. (2015) noting the contribution of CINAHL in identifying qualitative studies [ 76 ], Beckles et al. (2013) questioning the contribution of CINAHL to identifying clinical studies for guideline development [ 77 ], and Cooper et al. (2015) exploring the role of UK-focused bibliographic databases to identify UK-relevant studies [ 78 ]. The host of the database (e.g. OVID or ProQuest) has been shown to alter the search returns offered. Younger and Boddy [ 79 ] report differing search returns from the same database (AMED) but where the ‘host’ was different [ 79 ].

The average number of bibliographic database searched in systematic reviews has risen in the period 1994–2014 (from 1 to 4) [ 80 ] but there remains (as attested to by the guidance) no consensus on what constitutes an acceptable number of databases searched [ 48 ]. This is perhaps because thinking about the number of databases searched is the wrong question, researchers should be focused on which databases were searched and why, and which databases were not searched and why. The discussion should re-orientate to the differential value of sources but researchers need to think about how to report this in studies to allow findings to be generalised. Bethel (2017) has proposed ‘search summaries’, completed by the literature searcher, to record where included studies were identified, whether from database (and which databases specifically) or supplementary search methods [ 81 ]. Search summaries document both yield and accuracy of searches, which could prospectively inform resource use and decisions to search or not to search specific databases in topic areas. The prospective use of such data presupposes, however, that past searches are a potential predictor of future search performance (i.e. that each topic is to be considered representative and not unique). In offering a body of practice, this data would be of greater practicable use than current studies which are considered as little more than individual case studies [ 82 , 83 , 84 , 85 , 86 , 87 , 88 , 89 , 90 ].

When to database search is another question posed in the literature. Beyer et al. [ 91 ] report that databases can be prioritised for literature searching which, whilst not addressing the question of which databases to search, may at least bring clarity as to which databases to search first [ 91 ]. Paradoxically, this links to studies that suggest PubMed should be searched in addition to MEDLINE (OVID interface) since this improves the currency of systematic reviews [ 92 , 93 ]. Cooper et al. (2017) have tested the idea of database searching not as a primary search method (as suggested in the guidance) but as a supplementary search method in order to manage the volume of studies identified for an environmental effectiveness systematic review. Their case study compared the effectiveness of database searching versus a protocol using supplementary search methods and found that the latter identified more relevant studies for review than searching bibliographic databases [ 94 ].

Key stage six: Determining the process of literature searching and deciding where to search (supplementary search methods)

Table 2 also summaries the process of literature searching which follows bibliographic database searching. As Table 2 sets out, guidance that supplementary literature search methods should be used in systematic reviews recurs across documents, but the order in which these methods are used, and the extent to which they are used, varies. We noted inconsistency in the labelling of supplementary search methods between guidance documents.

Rather than focus on the guidance on how to use the methods (which has been summarised in a recent review [ 95 ]), we focus on the aim or purpose of supplementary search methods.

The Cochrane Handbook reported that ‘efforts’ to identify unpublished studies should be made [ 9 ]. Four guidance documents [ 2 , 3 , 6 , 9 ] acknowledged that searching beyond bibliographic databases was necessary since ‘databases are not the only source of literature’ [ 2 ]. Only one document reported any guidance on determining when to use supplementary methods. The IQWiG handbook reported that the use of handsearching (in their example) could be determined on a ‘case-by-case basis’ which implies that the use of these methods is optional rather than mandatory. This is in contrast to the guidance (above) on bibliographic database searching.

The issue for supplementary search methods is similar in many ways to the issue of searching bibliographic databases: demonstrating value. The purpose and contribution of supplementary search methods in systematic reviews is increasingly acknowledged [ 37 , 61 , 62 , 96 , 97 , 98 , 99 , 100 , 101 ] but understanding the value of the search methods to identify studies and data is unclear. In a recently published review, Cooper et al. (2017) reviewed the literature on supplementary search methods looking to determine the advantages, disadvantages and resource implications of using supplementary search methods [ 95 ]. This review also summarises the key guidance and empirical studies and seeks to address the question on when to use these search methods and when not to [ 95 ]. The guidance is limited in this regard and, as Table 2 demonstrates, offers conflicting advice on the order of searching, and the extent to which these search methods should be used in systematic reviews.

Key stage seven: Managing the references

Five of the documents provided guidance on managing references, for example downloading, de-duplicating and managing the output of literature searches [ 2 , 4 , 6 , 8 , 10 ]. This guidance typically itemised available bibliographic management tools rather than offering guidance on how to use them specifically [ 2 , 4 , 6 , 8 ]. The CEE handbook provided guidance on importing data where no direct export option is available (e.g. web-searching) [ 10 ].

The literature on using bibliographic management tools is not large relative to the number of ‘how to’ videos on platforms such as YouTube (see for example [ 102 ]). These YouTube videos confirm the overall lack of ‘how to’ guidance identified in this study and offer useful instruction on managing references. Bramer et al. set out methods for de-duplicating data and reviewing references in Endnote [ 103 , 104 ] and Gall tests the direct search function within Endnote to access databases such as PubMed, finding a number of limitations [ 105 ]. Coar et al. and Ahmed et al. consider the role of the free-source tool, Zotero [ 106 , 107 ]. Managing references is a key administrative function in the process of review particularly for documenting searches in PRISMA guidance.

Key stage eight: Documenting the search

The Cochrane Handbook was the only guidance document to recommend a specific reporting guideline: Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) [ 9 ]. Six documents provided guidance on reporting the process of literature searching with specific criteria to report [ 3 , 4 , 6 , 8 , 9 , 10 ]. There was consensus on reporting: the databases searched (and the host searched by), the search strategies used, and any use of limits (e.g. date, language, search filters (The CRD handbook called for these limits to be justified [ 6 ])). Three guidance documents reported that the number of studies identified should be recorded [ 3 , 6 , 10 ]. The number of duplicates identified [ 10 ], the screening decisions [ 3 ], a comprehensive list of grey literature sources searched (and full detail for other supplementary search methods) [ 8 ], and an annotation of search terms tested but not used [ 4 ] were identified as unique items in four documents.

The Cochrane Handbook was the only guidance document to note that the full search strategies for each database should be included in the Additional file 1 of the review [ 9 ].

All guidance documents should ultimately deliver completed systematic reviews that fulfil the requirements of the PRISMA reporting guidelines [ 108 ]. The guidance broadly requires the reporting of data that corresponds with the requirements of the PRISMA statement although documents typically ask for diverse and additional items [ 108 ]. In 2008, Sampson et al. observed a lack of consensus on reporting search methods in systematic reviews [ 109 ] and this remains the case as of 2017, as evidenced in the guidance documents, and in spite of the publication of the PRISMA guidelines in 2009 [ 110 ]. It is unclear why the collective guidance does not more explicitly endorse adherence to the PRISMA guidance.

Reporting of literature searching is a key area in systematic reviews since it sets out clearly what was done and how the conclusions of the review can be believed [ 52 , 109 ]. Despite strong endorsement in the guidance documents, specifically supported in PRISMA guidance, and other related reporting standards too (such as ENTREQ for qualitative evidence synthesis, STROBE for reviews of observational studies), authors still highlight the prevalence of poor standards of literature search reporting [ 31 , 110 , 111 , 112 , 113 , 114 , 115 , 116 , 117 , 118 , 119 ]. To explore issues experienced by authors in reporting literature searches, and look at uptake of PRISMA, Radar et al. [ 120 ] surveyed over 260 review authors to determine common problems and their work summaries the practical aspects of reporting literature searching [ 120 ]. Atkinson et al. [ 121 ] have also analysed reporting standards for literature searching, summarising recommendations and gaps for reporting search strategies [ 121 ].

One area that is less well covered by the guidance, but nevertheless appears in this literature, is the quality appraisal or peer review of literature search strategies. The PRESS checklist is the most prominent and it aims to develop evidence-based guidelines to peer review of electronic search strategies [ 5 , 122 , 123 ]. A corresponding guideline for documentation of supplementary search methods does not yet exist although this idea is currently being explored.

How the reporting of the literature searching process corresponds to critical appraisal tools is an area for further research. In the survey undertaken by Radar et al. (2014), 86% of survey respondents (153/178) identified a need for further guidance on what aspects of the literature search process to report [ 120 ]. The PRISMA statement offers a brief summary of what to report but little practical guidance on how to report it [ 108 ]. Critical appraisal tools for systematic reviews, such as AMSTAR 2 (Shea et al. [ 124 ]) and ROBIS (Whiting et al. [ 125 ]), can usefully be read alongside PRISMA guidance, since they offer greater detail on how the reporting of the literature search will be appraised and, therefore, they offer a proxy on what to report [ 124 , 125 ]. Further research in the form of a study which undertakes a comparison between PRISMA and quality appraisal checklists for systematic reviews would seem to begin addressing the call, identified by Radar et al., for further guidance on what to report [ 120 ].

Limitations

Other handbooks exist.

A potential limitation of this literature review is the focus on guidance produced in Europe (the UK specifically) and Australia. We justify the decision for our selection of the nine guidance documents reviewed in this literature review in section “ Identifying guidance ”. In brief, these nine guidance documents were selected as the most relevant health care guidance that inform UK systematic reviewing practice, given that the UK occupies a prominent position in the science of health information retrieval. We acknowledge the existence of other guidance documents, such as those from North America (e.g. the Agency for Healthcare Research and Quality (AHRQ) [ 126 ], The Institute of Medicine [ 127 ] and the guidance and resources produced by the Canadian Agency for Drugs and Technologies in Health (CADTH) [ 128 ]). We comment further on this directly below.

The handbooks are potentially linked to one another

What is not clear is the extent to which the guidance documents inter-relate or provide guidance uniquely. The Cochrane Handbook, first published in 1994, is notably a key source of reference in guidance and systematic reviews beyond Cochrane reviews. It is not clear to what extent broadening the sample of guidance handbooks to include North American handbooks, and guidance handbooks from other relevant countries too, would alter the findings of this literature review or develop further support for the process model. Since we cannot be clear, we raise this as a potential limitation of this literature review. On our initial review of a sample of North American, and other, guidance documents (before selecting the guidance documents considered in this review), however, we do not consider that the inclusion of these further handbooks would alter significantly the findings of this literature review.

This is a literature review

A further limitation of this review was that the review of published studies is not a systematic review of the evidence for each key stage. It is possible that other relevant studies could help contribute to the exploration and development of the key stages identified in this review.

This literature review would appear to demonstrate the existence of a shared model of the literature searching process in systematic reviews. We call this model ‘the conventional approach’, since it appears to be common convention in nine different guidance documents.

The findings reported above reveal eight key stages in the process of literature searching for systematic reviews. These key stages are consistently reported in the nine guidance documents which suggests consensus on the key stages of literature searching, and therefore the process of literature searching as a whole, in systematic reviews.

In Table 2 , we demonstrate consensus regarding the application of literature search methods. All guidance documents distinguish between primary and supplementary search methods. Bibliographic database searching is consistently the first method of literature searching referenced in each guidance document. Whilst the guidance uniformly supports the use of supplementary search methods, there is little evidence for a consistent process with diverse guidance across documents. This may reflect differences in the core focus across each document, linked to differences in identifying effectiveness studies or qualitative studies, for instance.

Eight of the nine guidance documents reported on the aims of literature searching. The shared understanding was that literature searching should be thorough and comprehensive in its aim and that this process should be reported transparently so that that it could be reproduced. Whilst only three documents explicitly link this understanding to minimising bias, it is clear that comprehensive literature searching is implicitly linked to ‘not missing relevant studies’ which is approximately the same point.

Defining the key stages in this review helps categorise the scholarship available, and it prioritises areas for development or further study. The supporting studies on preparing for literature searching (key stage three, ‘preparation’) were, for example, comparatively few, and yet this key stage represents a decisive moment in literature searching for systematic reviews. It is where search strategy structure is determined, search terms are chosen or discarded, and the resources to be searched are selected. Information specialists, librarians and researchers, are well placed to develop these and other areas within the key stages we identify.

This review calls for further research to determine the suitability of using the conventional approach. The publication dates of the guidance documents which underpin the conventional approach may raise questions as to whether the process which they each report remains valid for current systematic literature searching. In addition, it may be useful to test whether it is desirable to use the same process model of literature searching for qualitative evidence synthesis as that for reviews of intervention effectiveness, which this literature review demonstrates is presently recommended best practice.

Abbreviations

Behaviour of interest; Health context; Exclusions; Models or Theories

Cochrane Database of Systematic Reviews

The Cochrane Central Register of Controlled Trials

Database of Abstracts of Reviews of Effects

Enhancing transparency in reporting the synthesis of qualitative research

Institute for Quality and Efficiency in Healthcare

National Institute for Clinical Excellence

Population, Intervention, Comparator, Outcome

Preferred Reporting Items for Systematic Reviews and Meta-Analyses

Setting, Perspective, Intervention, Comparison, Evaluation

Sample, Phenomenon of Interest, Design, Evaluation, Research type

STrengthening the Reporting of OBservational studies in Epidemiology

Trial Search Co-ordinators

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Acknowledgements

CC acknowledges the supervision offered by Professor Chris Hyde.

This publication forms a part of CC’s PhD. CC’s PhD was funded through the National Institute for Health Research (NIHR) Health Technology Assessment (HTA) Programme (Project Number 16/54/11). The open access fee for this publication was paid for by Exeter Medical School.

RG and NB were partially supported by the National Institute for Health Research (NIHR) Collaboration for Leadership in Applied Health Research and Care South West Peninsula.

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CC conceived the idea for this study and wrote the first draft of the manuscript. CC discussed this publication in PhD supervision with AB and separately with JVC. CC revised the publication with input and comments from AB, JVC, RG and NB. All authors revised the manuscript prior to submission. All authors read and approved the final manuscript.

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Appendix tables and PubMed search strategy. Key studies used for pearl growing per key stage, working data extraction tables and the PubMed search strategy. (DOCX 30 kb)

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Cooper, C., Booth, A., Varley-Campbell, J. et al. Defining the process to literature searching in systematic reviews: a literature review of guidance and supporting studies. BMC Med Res Methodol 18 , 85 (2018). https://doi.org/10.1186/s12874-018-0545-3

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Systematic reviews

How to document your search.

Your search methodology should document where you looked for information and how many results were found.

Keep track of your activities as you search. It is much harder after the event to justify the decisions you made and to remember what you found in each source.

Consider using our search activity template (DOCX) as a personal record.

What you document depends on your reason for searching the literature. If you are carrying out detailed research for a systematic review you will probably need to provide rigorous documentation of your search process as part of your submission.

If you are unsure, check with your department in case there are local procedures you should be following.

Writing up your search methodology

A search methodology should document your search so that someone else can reproduce your steps and get the same results. Include:

• the names of the sources you search and which provider you accessed them through - eg Medline (Ovid), Web of Science (Thomson Reuters)
• any grey literature sources you used
• the date you carried out the searches
• any search limits you applied eg language, date ranges of publication, types of publication
• any individuals or organisations you contacted
• any sources you handsearched

For more detail on the search steps listed above go to our literature searching guide .

To see how you might write up a search methodology, the Cochrane Library has a number of good examples. Search for a systematic review and take a look at how the Methods section has been reported.

Add your search strategy as an appendix

The search strategies that you applied when searching different sources (eg Medline, Web of Science) can be added as an appendix to your document. This will provide the reader with additional detail on:

• how you searched (keyword and/or subject headings)
• which search terms you used (which words and phrases)
• any search techniques you employed (truncation, adjacency, etc)
• how you combined your search terms (AND/OR).

You can document in your results section the number of results you found in each source.

The following examples show how you could present a search strategy as an appendix:

• simple search strategy example (PDF)
• detailed search strategy example (PDF) .

It is often easier to copy and paste the search history straight from the database rather than retyping it.

Search tip: Many databases allow you to save your search strategies inside a free personal account area. We recommend that you do this. See our advice on saving your search for more information.

Further help

For more help with reporting systematic research, you can refer to the PRISMA website and to their PRISMA flow diagram (under Key Documents) which shows the process of a systematic search.

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Literature Search: Databases and Gray Literature

The literature search.

• A systematic review search includes a search of databases, gray literature, personal communications, and a handsearch of high impact journals in the related field.  See our list of recommended databases and gray literature sources on this page.
• a comprehensive literature search can not be dependent on a single database, nor on bibliographic databases only.
• inclusion of multiple databases helps avoid publication bias (georaphic bias or bias against publication of negative results).
• The Cochrane Collaboration recommends PubMed, Embase and the Cochrane Central Register of Controlled Trials (CENTRAL) at a minimum.     
• NOTE:  The Cochrane Collaboration and the IOM recommend that the literature search be conducted by librarians or persons with extensive literature search experience. Please contact the NIH Librarians for assistance with the literature search component of your systematic review. 

Cochrane Library

A collection of six databases that contain different types of high-quality, independent evidence to inform healthcare decision-making. Search the Cochrane Central Register of Controlled Trials here.

European database of biomedical and pharmacologic literature.

PubMed comprises more than 21 million citations for biomedical literature from MEDLINE, life science journals, and online books.

Largest abstract and citation database of peer-reviewed literature and quality web sources. Contains conference papers.

Web of Science

World's leading citation databases. Covers over 12,000 of the highest impact journals worldwide, including Open Access journals and over 150,000 conference proceedings. Coverage in the sciences, social sciences, arts, and humanities, with coverage to 1900.

Subject Specific Databases

APA PsycINFO

Over 4.5 million abstracts of peer-reviewed literature in the behavioral and social sciences. Includes conference papers, book chapters, psychological tests, scales and measurement tools.

CINAHL Plus

Comprehensive journal index to nursing and allied health literature, includes books, nursing dissertations, conference proceedings, practice standards and book chapters.

Latin American and Caribbean health sciences literature database

Gray Literature

• Gray Literature  is the term for information that falls outside the mainstream of published journal and mongraph literature, not controlled by commercial publishers
• hard to find studies, reports, or dissertations
• conference abstracts or papers
• governmental or private sector research
• clinical trials - ongoing or unpublished
• experts and researchers in the field     
• Library catalogs
• Professional association websites
• Google Scholar  - Search scholarly literature across many disciplines and sources, including theses, books, abstracts and articles.
• Dissertation Abstracts - dissertation and theses database - NIH Library biomedical librarians can access and search for you.
• NTIS  - central resource for government-funded scientific, technical, engineering, and business related information.
• AHRQ  - agency for healthcare research and quality
• Open Grey  - system for information on grey literature in Europe. Open access to 700,000 references to the grey literature.
• World Health Organization  - providing leadership on global health matters, shaping the health research agenda, setting norms and standards, articulating evidence-based policy options, providing technical support to countries and monitoring and assessing health trends.
• New York Academy of Medicine Grey Literature Report  - a bimonthly publication of The New York Academy of Medicine (NYAM) alerting readers to new gray literature publications in health services research and selected public health topics. NOTE: Discontinued as of Jan 2017, but resources are still accessible.
• Gray Source Index
• OpenDOAR - directory of academic repositories
• International Clinical Trials Registery Platform  - from the World Health Organization
• Australian New Zealand Clinical Trials Registry
• Brazilian Clinical Trials Registry
• Chinese Clinical Trial Registry - 
• ClinicalTrials.gov   - U.S.  and international federally and privately supported clinical trials registry and results database
• Clinical Trials Registry  - India
• EU clinical Trials Register
• Japan Primary Registries Network  
• Pan African Clinical Trials Registry

Performing Effective Literature Searches

For MDR and Other Regulations

Any medical device must have adequate clinical evidence to demonstrate compliance with relevant, essential safety and performance requirements. A thorough and systematic Literature Search is one of the most important components of a full and complete Clinical Evaluation . An SLR helps find appropriate clinical data that isn’t available from the manufacturer and demonstrates its clinical safety and performance.

The Literature Search establishes the safety and performance of the device, and also includes a review of the current knowledge and/or state of the art (SOTA). These two sections work together to help the device evidence its clinical safety and performance.

When limited data from the manufacturer is available, clinical data from scientific literature of equivalent/similar products can be a substantial portion of conformity evidence.

Vital Components

We should include all the sources of data, the methodology intended for each search, and the specific search words and criteria used n the Literature Search protocol. It’s crucial to determine how to apply selection criteria to published literature and justify why you’re doing so, as well as strategies for dealing with data duplication from multiple sources.

To maintain data integrity, manufacturers must define data management methods, such as quality control and a second evaluation of extracted data by additional reviewers. It is necessary to provide a rationale for an appraisal plan that includes methods for evaluating each of the data sources used and their relevance to their device. The Literature Search review report should also cover the analysis plan and define the methods for analysing the data, such as data processing and transformation.

Manufacturers must consult applicable standards and guidance documents in the corresponding medical device field to determine the current SOTA . Using information on the medical condition managed with the device, benchmark devices, and medical alternatives available to the target population would be beneficial.

The literature search, however, is complex, and the current guidelines require the search strategy to be thorough, objective, comprehensive, and repeatable, as well as documented and justified. The use of trained literature search engines that improve search results and save the manufacturer time could be advantageous.

Plan a Literature Search and Review Plan

Manufacturers should define a literature search protocol that scopes out the plan, clarifies any problems, and conforms with existing guidelines before beginning the search.

You must set a literature search plan with several searching approaches out before beginning the search. One of the most common methodologies is the PICO (P- Population, I- Intervention, C- Control or Comparison, O- Outcome). Other examples are  SPIDER, SPICE, PEO and ECLIPSE.  PRISMA statement is used as a reporting structure for your report. Where the approasial criteria can come from MEDDEV2.7.1rev4 or IMDRF Clinical Evaluation guidance, alternatively you can use CASP, MOOSE, CONSORT, STROBE, STARD, GRADE, NOS, PROBLAST, QADRAS, LEGEND or SPIRIT where appropriate.  

It’s vital for manufacturers to build a solid literature search technique that we can readily repeat and validate throughout future CER revision.

Identify the Most Suitable Literature Search Databases

The Literature Search database determines the quality of the data. Thus scientific databases are required, and not ‘grey’ databases such as internet searches, non-published data, and citations. CER’s commonly use scientific databases such as Cochrane, EMBASE, Pubmed, CHINAHL, SCOPUS, Cochrane Library or  PROQUEST  databases for literature search. 

Separately, vigilance databases from competent authorities provide real world evidence (RWE) in safety and performance, such as device adverse events and recalls. In future, EUDAMED will be the database for EU MDR but it doesn’t have the critical volume of data other sites do yet. Therefore, searches on competent authority websites such as the FDA, BfArM, Sante, MHRA, HPRA etc are being used.  

Start Systematic Literature Review

For the best results, you should use a medical librarian or experienced SLR reviewer to carefully develop your search syntax. The syntax is made up from chosen keywords that have been vetted for reach and strategies, such as Boolean operators. This is designed to ensure you find articles relevant to your device. You should also use filters to refine results to meet the expected criteria. We should check various searches  for the most relevant and best reach before signing off on the search strategy. 

Conduct your literature search and transfer all your results into your inclusion and appraisal workflow. Articles will need to be screened one by one to determine their relevance. Where there is doubt, a second round of screening needs to be done based on inclusion and exclusion criteria, which will consider the full text article. Once this is done, you now have your articles to review for their suitability and contribution. 

Suitability appraisal explores whether it is the correct type of device, the same Intended purpose of the device, and for what population and indication it was used in the article. I t’s vital to determine if the data you’ve obtained is accurate and relevant to the medical device being evaluated.  

Contribution appraisal looks at the quality of the data and design of the study.  The MEDDEV guidelines provide guidance on evaluating data sets’ consistency, methodological quality, and scientific validity. 

The appraisal criteria does not cover the risk of bias, so you need to include it in your SLR process. Bias has a big impact on the reliability and credibility of the data.  

Prepare the Systematic Literature Review (SLR) Report

Each review must produce an SLR report. This can either be a stand-alone document (recommended) or reported in the CER. 

The report will use the results that have passed through each reviewer’s checks of the inclusion criteria and appraisals. 

The SLR needs to contain a summary of each article, such as the type of study (RCT, Meta-analysis, cohort ext), type of device, indication and population. This summary is usually in a table format and can also cover points such as safety and performance, state of the art, benefits to the patient, along with side-effects and adverse reactions, etc. By putting the information in this format, it will also be easier to use in the CER to present evidence of GSPR conformity. 

Having a robust and comprehensive SLR template in place which outlines all the information and highlights key areas such as the process, databases searched, criteria, findings and conclusion is essential. 

CLIN-r+ Recommendations

To be successful, a literature search must be conducted by an experienced medical device or IVD systematic literature reviewer. There must be clearly defined protocols and well planned database search syntax the searches are conducted and the results placed into the SLR process. It is important to set out analytical and evaluation methods to be used, and ensure you have the specialised skills as stated in the MEDDEV 2.7.1rev4. All CER’s and their SLR reports must be conducted by staff that meet the MEDDEV2.7.1rev4 requirements to be accepted by auditors. 

The SLR process must be conducted in a systematic way with evidence to show the process has been adhered to. Staying within the Clinical Evaluation Plan’s device search parameters is crucial.

It is important to note that we should keep the literature search separate from the State of the Art ( SOTA ) search. E.g., once we have identified known papers and sales literature comparative to the device, it is still necessary to further test the search parameters and ensure that references to the papers and sales literature appear in the SLR search results.

Overall, the literature search must be exceedingly thorough, consistent, and cover a wide range of search parameters. We must objectively document the findings in detail, and include good, bad and justifiable results. Additionally, we must ensure that it can be easily replicated and independently verified.

If you have further questions and would like to discuss how we can support you in the systematic literature review process,  get in touch .

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• Published: 18 April 2024

Hydrogen inhalation therapy for inflammation and eye diseases: a review of the literature

• Yoshiyasu Takefuji   ORCID: orcid.org/0000-0002-1826-742X 1  

Eye ( 2024 ) Cite this article

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An exhaustive survey of existing literature was conducted to investigate the relationship between hydrogen inhalation therapy, inflammation, and ocular disordhers. The burgeoning body of evidence indicates that hydrogen inhalation therapy might be a promising approach to alleviate not only inflammation but also related eye diseases.

Liang et al. investigated the effect of hydrogen gas inhalation on age-related macular degeneration in mice [ 1 ]. Hydrogen gas, an antioxidant, was found to reduce leakage in choroidal neovascularization, a key feature of the disease. The treatment also suppressed inflammation and the expression of vascular endothelial growth factor, potentially inhibiting disease progression [ 1 ].

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Liang IC, Ko WC, Hsu YJ, Lin YR, Chang YH, Zong XH, et al. The anti-inflammatory effect of hydrogen gas inhalation and its influence on laser-induced choroidal neovascularization in a mouse model of neovascular age-related macular degeneration. Int J Mol Sci. 2021;22:12049. https://doi.org/10.3390/ijms222112049 .

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Li SY, Xue RY, Wu H, Pu N, Wei D, Zhao N, et al. Novel role of molecular hydrogen: the end of ophthalmic diseases? Pharmaceuticals. 2023;16:1567. https://doi.org/10.3390/ph16111567 .

Otsuka M, Arai K, Yoshida T, Hayashi A. Correction: Inhibition of retinal ischemia-reperfusion injury in rats by inhalation of low-concentration hydrogen gas. Graefes Arch Clin Exp Ophthalmol. 2023. https://doi.org/10.1007/s00417-023-06340-6 .

Artamonov MY, Martusevich AK, Pyatakovich FA, Minenko IA, Dlin SV, LeBaron TW. Molecular hydrogen: from molecular effects to stem cells management and tissue regener''ation. Antioxidants. 2023;12:636. https://doi.org/10.3390/antiox12030636 .

Johnsen HM, Hiorth M, Klaveness J. Molecular hydrogen therapy-a review on clinical studies and outcomes. Molecules. 2023;28:7785. https://doi.org/10.3390/molecules28237785 .

Cheng D, Long J, Zhao L, Liu J. Hydrogen: a rising star in gas medicine as a mitochondria-targeting nutrient via activating keap1-Nrf2 antioxidant system. Antioxidants. 2023;12:2062. https://doi.org/10.3390/antiox12122062 .

Cejka C, Kubinova S, Cejkova J. The preventive and therapeutic effects of molecular hydrogen in ocular diseases and injuries where oxidative stress is involved. Free Radic Res. 2019;53:237–47. https://doi.org/10.1080/10715762.2019.1582770 .

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Zhang ZQ, Xie Z, Chen SY, Zhang X. Mitochondrial dysfunction in glaucomatous degeneration. Int J Ophthalmol. 2023;16:811–23. https://doi.org/10.18240/ijo.2023.05.20 .

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Perveen I, Bukhari B, Najeeb M, Nazir S, Faridi TA, Farooq M, et al. Hydrogen therapy and its future prospects for ameliorating COVID-19: clinical applications, efficacy, and modality. Biomedicines. 2023;11:1892. https://doi.org/10.3390/biomedicines11071892 .

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Takefuji, Y. Hydrogen inhalation therapy for inflammation and eye diseases: a review of the literature. Eye (2024). https://doi.org/10.1038/s41433-024-03083-4

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DOI : https://doi.org/10.1038/s41433-024-03083-4

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• Indian J Anaesth
• v.60(9); 2016 Sep

Literature search for research planning and identification of research problem

Anju grewal.

Department of Anaesthesiology, Dayanand Medical College and Hospital, Ludhiana, Punjab, India

Hanish Kataria

1 Department of Surgery, Government Medical College and Hospital, Chandigarh, India

2 Department of Cardiac Anaesthesia, All India Institute of Medical Sciences, New Delhi, India

Literature search is a key step in performing good authentic research. It helps in formulating a research question and planning the study. The available published data are enormous; therefore, choosing the appropriate articles relevant to your study in question is an art. It can be time-consuming, tiring and can lead to disinterest or even abandonment of search in between if not carried out in a step-wise manner. Various databases are available for performing literature search. This article primarily stresses on how to formulate a research question, the various types and sources for literature search, which will help make your search specific and time-saving.

INTRODUCTION

Literature search is a systematic and well-organised search from the already published data to identify a breadth of good quality references on a specific topic.[ 1 ] The reasons for conducting literature search are numerous that include drawing information for making evidence-based guidelines, a step in the research method and as part of academic assessment.[ 2 ] However, the main purpose of a thorough literature search is to formulate a research question by evaluating the available literature with an eye on gaps still amenable to further research.

Research problem[ 3 ] is typically a topic of interest and of some familiarity to the researcher. It needs to be channelised by focussing on information yet to be explored. Once we have narrowed down the problem, seeking and analysing existing literature may further straighten out the research approach.

A research hypothesis[ 4 ] is a carefully created testimony of how you expect the research to proceed. It is one of the most important tools which aids to answer the research question. It should be apt containing necessary components, and raise a question that can be tested and investigated.

The literature search can be exhaustive and time-consuming, but there are some simple steps which can help you plan and manage the process. The most important are formulating the research questions and planning your search.

FORMULATING THE RESEARCH QUESTION

Literature search is done to identify appropriate methodology, design of the study; population sampled and sampling methods, methods of measuring concepts and techniques of analysis. It also helps in determining extraneous variables affecting the outcome and identifying faults or lacunae that could be avoided.

Formulating a well-focused question is a critical step for facilitating good clinical research.[ 5 ] There can be general questions or patient-oriented questions that arise from clinical issues. Patient-oriented questions can involve the effect of therapy or disease or examine advantage versus disadvantage for a group of patients.[ 6 ]

For example, we want to evaluate the effect of a particular drug (e.g., dexmedetomidine) for procedural sedation in day care surgery patients. While formulating a research question, one should consider certain criteria, referred as ‘FINER’ (F-Feasible, I-Interesting, N-Novel, E-Ethical, R-Relevant) criteria.[ 5 ] The idea should be interesting and relevant to clinical research. It should either confirm, refute or add information to already done research work. One should also keep in mind the patient population under study and the resources available in a given set up. Also the entire research process should conform to the ethical principles of research.

The patient or study population, intervention, comparison or control arm, primary outcome, timing of measurement of outcome (PICOT) is a well-known approach for framing a leading research question.[ 7 , 8 ] Dividing the questions into key components makes it easy and searchable. In this case scenario:

• Patients (P) – What is the important group of patients? for example, day care surgery
• Intervention (I) – What is the important intervention? for example, intravenous dexmedetomidine
• Comparison (C) – What is the important intervention of comparison? for example, intravenous ketamine
• Outcome (O) – What is the effect of intervention? for example, analgesic efficacy, procedural awareness, drug side effects
• Time (T) – Time interval for measuring the outcome: Hourly for first 4 h then 4 hourly till 24 h post-procedure.

Multiple questions can be formulated from patient's problem and concern. A well-focused question should be chosen for research according to significance for patient interest and relevance to our knowledge. Good research questions address the lacunae in available literature with an aim to impact the clinical practice in a constructive manner. There are limited outcome research and relevant resources, for example, electronic database system, database and hospital information system in India. Even when these factors are available, data about existing resources is not widely accessible.[ 9 ]

TYPES OF MEDICAL LITERATURE

(Further details in chapter ‘Types of studies and research design’ in this issue).

Primary literature

Primary sources are the authentic publication of an expert's new evidence, conclusions and proposals (case reports, clinical trials, etc) and are usually published in a peer-reviewed journal. Preliminary reports, congress papers and preprints also constitute primary literature.[ 2 ]

Secondary literature

Secondary sources are systematic review articles or meta-analyses where material derived from primary source literature are infererred and evaluated.[ 2 ]

Tertiary literature

Tertiary literature consists of collections that compile information from primary or secondary literature (eg., reference books).[ 2 ]

METHODS OF LITERATURE SEARCH

There are various methods of literature search that are used alone or in combination [ Table 1 ]. For past few decades, searching the local as well as national library for books, journals, etc., was the usual practice and still physical literature exploration is an important component of any systematic review search process.[ 10 , 11 ] With the advancement of technology, the Internet is now the gateway to the maze of vast medical literature.[ 12 ] Conducting a literature review involves web-based search engines, i.e., Google, Google Scholar, etc., [ Table 2 ], or using various electronic research databases to identify materials that describe the research topic or those homologous to it.[ 13 , 14 ]

Methods of literature search

Web based methods of literature search

The various databases available for literature search include databases for original published articles in the journals [ Table 2 ] and evidence-based databases for integrated information available as systematic reviews and abstracts [ Table 3 ].[ 12 , 14 ] Most of these are not freely available to the individual user. PubMed ( http://www.ncbi.nlm.nih.gov/pubmed/ ) is the largest available resource since 1996; however, a large number of sources now provide free access to literature in the biomedical field.[ 15 ] More than 26 million citations from Medline, life science journals and online books are included in PubMed. Links to the full-text material are included in citations from PubMed Central and publisher web sites.[ 16 ] The choice of databases depends on the subject of interest and potential coverage by the different databases. Education Resources Information Centre is a free online digital library of education research and information sponsored by the Institute of Education Sciences of the U.S. Department of Education, available at http://eric.ed.gov/ . No one database can search all the medical literature. There is need to search several different databases. At a minimum, PubMed or Medline, Embase and the Cochrane central trials Registry need to be searched. When searching these databases, emphasis should be given to meta-analysis, systematic reviews randomised controlled trials and landmark studies.

Electronic source of Evidence-Based Database

Time allocated to the search needs attention as exploring and selecting data are early steps in the research method and research conducted as part of academic assessment have narrow timeframes.[ 17 ] In Indian scenario, limited outcome research and accessibility to data leads to less thorough knowledge of nature of research problem. This results in the formulation of the inappropriate research question and increases the time to literature search.

TYPES OF SEARCH

Type of search can be described in different forms according to the subject of interest. It increases the chances of retrieving relevant information from a search.

Translating research question to keywords

This will provide results based on any of the words specified; hence, they are the cornerstone of an effective search. Synonyms/alternate terms should be considered to elicit further information, i.e., barbiturates in place of thiopentone. Spellings should also be taken into account, i.e., anesthesia in place of anaesthesia (American and British). Most databases use controlled word-stock to establish common search terms (or keywords). Some of these alternative keywords can be looked from database thesaurus.[ 4 ] Another strategy is combining keywords with Boolean operators. It is important to keep a note of keywords and methods used in exploring the literature as these will need to be described later in the design of search process.

‘Medical Subject Heading (MeSH) is the National Library of Medicine's controlled hierarchical vocabulary that is used for indexing articles in PubMed, with more specific terms organised underneath more general terms’.[ 17 ] This provides a reliable way to retrieve citations that use different terminology for identical ideas, as it indexes articles based on content. Two features of PubMed that can increase yield of specific articles are ‘Automatic term mapping’ and ‘automatic term explosion’.[ 4 ]

For example, if the search keyword is heart attack, this term will match with MeSH transcription table heading and then explode into various subheadings. This helps to construct the search by adding and selecting MeSH subheadings and families of MeSH by use of hyperlinks.[ 4 ]

We can set limits to a clinical trial for retrieving higher level of evidence (i.e., randomised controlled clinical trial). Furthermore, one can browse through the link entitled ‘Related Articles’. This PubMed feature searches for similar citations using an intricate algorithm that scans titles, abstracts and MeSH terms.[ 4 ]

Phrase search

This will provide pages with only the words typed in the phrase, in that exact order and with no words in between them.

Boolean operators

AND, OR and NOT are the three Boolean operators named after the mathematician George Boole.[ 18 ] Combining two words using ‘AND’ will fetch articles that mention both the words. Using ‘OR’ will widen the search and fetch more articles that mention either subject. While using the term ‘NOT’ to combine words will fetch articles containing the first word but not the second, thus narrowing the search.

Filters can also be used to refine the search, for example, article types, text availability, language, age, sex and journal categories.

Overall, the recommendations for methodology of literature search can be as below (Creswell)[ 19 ]

• Identify keywords and use them to search articles from library and internet resources as described above
• Search several databases to search articles related to your topic
• Use thesaurus to identify terms to locate your articles
• Find an article that is similar to your topic; then look at the terms used to describe it, and use them for your search
• Use databases that provide full-text articles (free through academic libraries, Internet or for a fee) as much as possible so that you can save time searching for your articles
• If you are examining a topic for the first time and unaware of the research on it, start with broad syntheses of the literature, such as overviews, summaries of the literature on your topic or review articles
• Start with the most recent issues of the journals, and look for studies about your topic and then work backward in time. Follow-up on references at the end of the articles for more sources to examine
• Refer books on a single topic by a single author or group of authors or books that contain chapters written by different authors
• Next look for recent conference papers. Often, conference papers report the latest research developments. Contact authors of pertinent studies. Write or phone them, asking if they know of studies related to your area of interest
• The easy access and ability to capture entire articles from the web make it attractive. However, check these articles carefully for authenticity and quality and be cautious about whether they represent systematic research.

The whole process of literature search[ 20 ] is summarised in Figure 1 .

Process of literature search

Literature search provides not only an opportunity to learn more about a given topic but provides insight on how the topic was studied by previous analysts. It helps to interpret ideas, detect shortcomings and recognise opportunities. In short, systematic and well-organised research may help in designing a novel research.

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Title: related work and citation text generation: a survey.

Abstract: To convince readers of the novelty of their research paper, authors must perform a literature review and compose a coherent story that connects and relates prior works to the current work. This challenging nature of literature review writing makes automatic related work generation (RWG) academically and computationally interesting, and also makes it an excellent test bed for examining the capability of SOTA natural language processing (NLP) models. Since the initial proposal of the RWG task, its popularity has waxed and waned, following the capabilities of mainstream NLP approaches. In this work, we survey the zoo of RWG historical works, summarizing the key approaches and task definitions and discussing the ongoing challenges of RWG.

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