literature review vs search

Educational resources and simple solutions for your research journey

Differences between literature search and literature review

You’ve just found out you need to do a literature review. You know the term literature search, but you’re not sure what the difference is between a literature search and a literature review. This can be confusing if you’re just starting out in academia. Although literature search and review are related terms, they refer to different processes and functions. In general, a literature search is the process of seeking out and identifying the existing literature related to a topic or question of interest, while a literature review is the organized synthesis of the information found in the existing literature.

In research applications, a literature search is typically the first step of a literature review. The search identifies relevant existing studies and articles, and the review is the end result of analyzing, synthesizing, and organizing the information found in the search. The following summarizes the literature search and review concepts as researchers typically practice them.

Literature search

A literature search is a systematic search for existing information on your question or topic 1 . The purpose of a literature search is to aid in the formulation of a research question and study design. When you are planning to conduct a study on a specific topic, the literature search helps narrow the focus of your study by identifying areas in which knowledge gaps exist. The search of existing studies can also guide the research design by suggesting appropriate methodologies and important variables. Research studies are never done in a vacuum – they are built on previous knowledge. A comprehensive literature search and review will provide you the base on which to build your study.

An effective search needs to be planned. Here are some tips for conducting a literature search 4 .

• Identify key words to use when searching through library and internet resources.
• Search multiple databases for relevant articles, books, and other scholarly writings.
• Use articles similar to your proposed study to find additional keywords.
• Start with the most recent articles and work backward in time if necessary.
• Include conference papers in your search as they generally represent the latest research.
• Cast a wide net by searching in databases that might be unrelated to your topic.
• Keep in mind that literature searches are iterative processes. Find new key words and articles through the references and citations in other relevant sources.
• Make sure to document all of the articles you identify as relevant to your topic. This will save you time and frustration later when you want to find them again and when you need to write references for your literature review.

In a literature review, the results of a literature search are used to produce an organized and coherent presentation of the relevant knowledge about a specific topic. This is accomplished through reviewing, evaluating, analyzing, and synthesizing the information found through a search. An effective literature review clearly places the proposed study in the context of previous research studies and identifies a gap in the knowledge that will be addressed by the proposed study.

A good literature review serves to demonstrate the depth of your knowledge and understanding of the topic; it is not simply a summary or description of those studies 2 . Here are some tips in conducting an effective literature review process.

• Identify a wide range of articles using a literature search.
• Evaluate those articles to determine which are relevant to your review 2 . When evaluating the research, include considerations such as the significance of the study, the methodology, the value of the analysis, the structure of the article, and the overall effectiveness of the study.
• Analyze the articles you’ve chosen to include. Critically and objectively review the study’s methods, results, and conclusions. Look for strengths and weaknesses. What can you learn from this study as it relates to your work?
• Synthesize the information from all of the included sources. Look for patterns in the articles. What do they agree on? What do they disagree on? What is missing from the information?
• Organize your literature review based on chronology, methodology, or themes. Again, this should not be merely a listing of the literature but a carefully structured whole.
• Write your literature review using the format of an introduction, body, and conclusion.

Additional tips for researchers

• Always strive for objectivity when conducting a literature search or review. Include all viewpoints and do not begin the process expecting a specific result. Avoid opinions.
• Make sure your selected sources and your literature review work to place your study in the context of the existing literature.
• The literature should reveal a knowledge gap that will be addressed by your study.
• As with all writing, keep your audience in mind.

Table of Contents

• Grewal A, Kataria H, Dhawan I. Literature search for research planning and identification of research problem. Indian J Anaesth. 2016, 60, 635-639. doi: 10.4103/0019-5049.190618.
• Niagara University Library Research Guide. Literature Review. https://niagara.libguides.com/litreview/sixsteps [Accessed August 31, 2022]

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Literature Searches and Literature Reviews: Getting Started

• Getting Started
• Literature Searches
• Literature Reviews
• Resources: Where to Search

What's in This Guide

► Getting Started

Definitions, featured resources and related guides.

► Literature Searches

A step-by-step guide and links to key search resources.

► Literature Reviews

► Resources: Where to Search

An extensive list of bibliographic databases, research databases, websites and other resources.

Related Resource Guides

Research Tools National Transportation Library

Hidden Treasures: Finding the Transportation Information You Need Oregon DOT

Recommended Resources in Transportation ETKN

Start Your Research Northwestern University

Steps for Conducting a Literature Review University of West Florida Libraries

Copyright for Research Oregon DOT

Literature searches and literature reviews are key components of research projects sponsored by state departments of transportation. A literature search seeks to confirm what is known about a particular topic, and a literature review uses the literature search to help to clarify the scope of the research project. This resource guide addresses:

• How to conduct literature searches.
• Where to search for transportation information.
• How to put it all together as a quality literature review.

The guide is adapted from  Transportation Research E-Circular 194: Literature Searches and Literature Reviews for Transportation Research Projects  (2015). For a more detailed discussion, review the literature search and literature review sections of the e-circular.

Practical Tips

• When conducting a literature search, keep in mind that information is available in numerous places. While running a search on Google may be a useful place to start, a wealth of additional resources, including databases, search engines, websites and catalogs, will provide valuable contributions to a thorough search effort.
• Remember to contact your librarian during the search. A librarian can help to identify databases and catalogs, and can often locate documents that aren't available online or in your library. 

Definitions

Literature search  — a series of searches across multiple resources that produces a list of citations which are relevant to a topic or subject matter. The results are compiled and structured to help a researcher understand the scope, breadth and relevance of the literature on a topic.

►  Example : Micro-Rail Exploration Study , California Department of Transportation, March 2021.

Literature review  — a narrative that is based on the findings of a literature search. The narrative is organized by topic, drawing connections among citations and presenting cited works according to their importance and relevance.

►  Example : Snow Removal at Extreme Temperatures , Clear Roads Pooled Fund, Minnesota Department of Transportation, March 2013. 

About the Pooled Fund

Featured Resource

Literature Searches and Literature Reviews for Transportation Research Projects , 2015.

Authors and Contributors

This Resource Guide was prepared by CTC & Associates LLC for the Transportation Research and Connectivity  pooled fund study, TPF-5(442), under the guidance of the following members of the study's technical advisory committee:

• Ned Parrish , Idaho Transportation Department
• Laura Wilt ,  Oregon DOT
• Michael Molina , Oklahoma Transportation Library (lead state technical contact)

This guide is a living document that is intended to be revised and updated to incorporate new resources. To suggest a resource for inclusion, please contact one of the committee members listed above.

Publication date: September 2021.

• Next: Literature Searches >>
• Last Updated: Aug 9, 2022 1:53 PM
• URL: https://transportation.libguides.com/Lit-Searches-Reviews

Will You Be a Catalyst for India's Medical Research Revolution?

Literature search & literature review – what is the difference?

It is interesting to note, that too many people confuse the terms “Literature Review” and “Literature Searching”. Long ago, I noticed this when if invited to deliver a lecture, then the topic against my name would be “Literature Review”. I needed to tell the organizers that I do not teach this, but I teach Literature Searching. Sometimes in spite of this when I was being introduced just before my talk, this term would appear again!

A literature search is the process of doing a structured, methodological search to fine the existing literature related on a topic for research, or even something one is just interested in learning more about. A literature review is the method used to demonstrate how much the author of an article has read on the topic and to synthesize the information found in the existing literature.

There are several steps in doing an effective, structured literature search. Once one retrieves search results, one needs to remove duplicates found from several databases, and then go through the results to select the papers to study for one’s own research.

Again there are several steps to carry out for writing the literature review. I found a simple blog post on both these aspects . Do go through this one to understand the basic steps involved

Then it is vital to learn literature searching in detail. This – whether you plan to do searches on your own or involve a librarian or information specialist to do them for you. Only when you give one of them the right inputs can the person do a search for you and get you the kind of results you need. QMed’s ELearning courses have been created for you to learn these skills!

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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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Conflicts of interest.

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• 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.

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: Oct 27, 2023 12:59 PM
• URL: https://libguides.und.edu/literature-reviews

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

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.

Different search strategies

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

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• Last Updated: Oct 12, 2023 3:36 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

185k Accesses

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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.

The views expressed are those of the author(s) and not necessarily those of the NHS, the NIHR or the Department of Health.

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Contributions

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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Additional file

Additional file 1:.

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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DOI : https://doi.org/10.1186/s12874-018-0545-3

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Writing a Literature Review: Search the Literature

• Select a Topic
• Search the Literature
• Read and Analyse the Literature
• Write the Review
• Referencing This link opens in a new window

A step by step guide

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

KEY DATABASES TO USE

• Australian Health Databases (Informit) This link opens in a new window Covers research in medicine, nursing, allied health and public health. Terms of Use
• CINAHL Complete This link opens in a new window Access journal articles on nursing and allied health research. Terms of Use
• Medline Complete (via EBSCO) This link opens in a new window Access journal articles from the largest biomedical and health research database in the world via the EBSCO platform. Terms of Use
• PsycINFO This link opens in a new window
• PubMed This link opens in a new window
• ScienceDirect This link opens in a new window

Search Strategy

A good literature review requires a comprehensive literature search, identifying all relevant research articles that best addresses your research question. It is important to structure your literature search systematically; clearly identify the scope before you begin and keep in mind the quality and relevance of the literature as you proceed.

Begin your search strategy by using the PICO framework below:

Structuring your question will clarify the search and identify keywords or search terms that will assist with your search.When conducting a search within a large database your search terms should be closely related to the components of your PICO question. For more information see the Tutorial on the right.

You want to conduct a literature review that asks the question:

For an adult with lower back pain, is exercise or resting a more effective treatment?

P -  Adult with lower back pain

I   -  Exercise

C -  Rest

O -  Relief of lower back pain

Using Synonyms

When conducting a systematic search for a literature review it is best to find all the synonyms you can on your keywords, such as lower back pain, lower backache

Join the synonyms together using OR eg. lower back pain OR lower backache

Joining Concepts Together for the Database Searching

Use AND to join the main concepts together

lower back pain OR lower backache

exercise OR physical activity

Using the Limits or Filters within the databases helps define the scope of the literature review - for example is the focus on children or all recent literature that has been published in the last 5 years.

For the above example a limit to "All Adults" can be used.

Build a Search using MeSH

MeSH is the National Library of Medicine's controlled vocabulary thesaurus. Headings within the thesaurus are arranged in a hierarchical structure known as a "tree" that enables searches to be conducted at various levels of specificity. MeSH thesaurus can help find words used by indexers for medical concepts. This thesaurus works in the background of Medline by linking synonym.

EBSCO Medline Search Example

Where to Search?

Conducting a thorough search to identify relevant studies is a key factor in minimizing bias in the review process. The search process should be as transparent as possible and documented in a way that enables it to be evaluated and reproduced.  It is best to search multiple databases when looking for research and to also document how you conduct your search in the Methods section of your literature review.

• Informit (Australian Health Databases)
• ScienceDirect
• Scanning reference lists of relevant studies
• Contacting study authors, experts, manufacturers, and other organisations
• Searching relevant Internet resources
• Searching Google Scholar to check "Cited by"
• Grey Literature

Write it Up!

• how you searched (e.g. keywords and/or subjects)
• search terms used (e.g. words and phrases)
• search techniques used (e.g. nesting, truncation, etc.)
• how you combined searches (e.g. AND / OR / NOT)

Source - http://research.library.gsu.edu/c.php?g=115595&p=754153

What are you going to include and exclude?

What articles you include and exclude is determined by the goals, coverage  and context of the research question. This inclusion / exclusion criteria must be made explicit in the Literature Review. For example, you might include or exclude according to:

1. The right Type of study for your review question? e.g. Randomized Control Trials; case studies,cohort studies

2. Publication timeframe e.g. 2006-2016

3. Population or patient group 

4. Significant sample size

5. Published in English

Moher, D., Shamseer, L., Clarke, M., Ghersi, D., Liberati, A., Petticrew, M., & ... Stewart, L. A. (2015). Preferred reporting items for systematic review and meta-analysis protocols (PRISMA-P) 2015 statement. Systematic Reviews, 41. doi: 10.1186/2046-4053-4-1

To generate a PRISMA flow diagram from a csv file use this handy tool created by Neal R Haddaway and Luke A McGuinness

•   Flow diagram generator

Flow charts can be referenced using the following citation:

Neal R Haddaway, Luke A McGuinness. (2020). PRISMA2020: R package and ShinyApp for producing PRISMA 2020 compliant flow diagrams (Version 0.0.1). Zenodo.  http://doi.org/10.5281/zenodo.4287835

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Literature searching and finding evidence

Literature searching or 'literature review'.

• Use the PICO or PEO frameworks
• Establish your Inclusion and Exclusion criteria
• Find related search terms
• Subject Heading/MeSH Searching
• Select databases to search
• Structure your search
• Search techniques
• Search key databases
• Manage results in EBSCOhost and Ovid
• Analyse your search results
• Document your search results
• Training and support

For assignments , projects and dissertations you will need to do a literature search:  find existing literature and information about your chosen topic to use as evidence in support of arguments or points you make.

You may also be asked to produce a literature review:  summarising and analysing relevant literature you have found through your research. In a literature review, the literature itself is the main subject of discussion. The aim of a literature review is to show you have identified and examined the key research on a particular topic or question.

• Am I the only one struggling to write a literature review? (Video) Watch this video which provides an overview of writing an effective literature review.

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