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Systematic Reviews

Document the Literature Search

The search process (including the sources searched, when, by whom, and using which terms) needs to be documented in enough detail throughout the process to ensure that it can be reported correctly in the review, to the extent that all the searches of all the databases are reproducible.

Items to document during the search process
Information sources & methods

Database name
Search date and searcher's name
Study registries used
Grey literature used
Any method used to find additional information

Search strategies

Full search strings
Search limits (such as publication date range or language)
Search filters used
Updates to the search (such as redoing the search at a later date or search alerts)

Managing records How search results were organized
Whether a reference manager was used
How results were de-duplicated

"Documenting the searching of sources other than databases, including the search terms used, is also required if searches are to be reproducible (Atkinson et al 2015, Chow 2015, Witkowski and Aldhouse 2015). Details about contacting experts or manufacturers, searching reference lists, scanning websites, and decisions about search iterations can be kept internally for future updates or external requests and can be reproduced as an appendix in the final document. Since the purpose of search documentation is to support transparency, internal assessment, and reference for any future update, it is important to plan how to record searching of sources other than databases since some activities (contacting experts, reference list searching, and forward citation searching) will occur later on in the review process after the database results have been screened (Rader et al 2014). The searcher should record any correspondence on key decisions and report a summary of this correspondence alongside the search strategy. The narrative describes the major decisions that shaped the strategy and can give a peer reviewer an insight into the rationale for the search approach (Craven and Levay 2011)."

Lefebvre C, Glanville J, Briscoe S, Littlewood A, Marshall C, Metzendorf M-I, Noel-Storr A, Rader T, Shokraneh F, Thomas J, Wieland LS. Chapter 4: Searching for and selecting studies. In: Higgins JPT, Thomas J, Chandler J, Cumpston M, Li T, Page MJ, Welch VA (editors). Cochrane Handbook for Systematic Reviews of Interventions version 6.1 (updated September 2020). Cochrane, 2020. Available from

Database Selection

A systematic review aims to capture as many relevant studies as possible as well as to minimize the bias, which can only be done by searching more than one database.  At the very minimum, at least the three databases should be used.

"Going beyond MEDLINE is important not only for ensuring that as many relevant studies as possible are identified, but also to minimize selection bias for those that are found. Relying exclusively on a MEDLINE search may retrieve a set of reports unrepresentative of all reports that would have been identified through a wider or more extensive search of several sources."

Lefebvre C, Glanville J, Briscoe S, Littlewood A, Marshall C, Metzendorf M-I, Noel-Storr A, Rader T, Shokraneh F, Thomas J, Wieland LS. Chapter 4: Searching for and selecting studies. In: Higgins JPT, Thomas J, Chandler J, Cumpston M, Li T, Page MJ, Welch VA (editors). Cochrane Handbook for Systematic Reviews of Interventions version 6.1 (updated September 2020). Cochrane, 2020. Available from


General Databases

Subject Specific Databases

Based on your topic, you may want to search subject specific databases.

Grey Literature

What is grey literature?

Examples of grey literature include: conference abstracts, presentations, proceedings; regulatory data; unpublished trial data; government publications; reports (such as white papers, working papers, internal documentation); dissertations/theses; patents;  and policies & procedures.

Why search grey literature?

Grey literature can be very useful, as it is information or research that may not have been published in a journal article or book.  Using grey literature mitigates "publication bias," as studies with positive results are more likely to be published than those showing no effects or negative effects of a drug or treatment.  Professional associations may have more in-depth or practical information on a topic than what you would find in journal articles.  By searching the grey literature, you are getting a complete picture. 

However, it is also important to remember that grey literature is not peer reviewed so it may not be of the highest quality.  It may not always be possible to judge the research methods used in unpublished briefings or reports.  The format of grey literature can be varied, so it may be more difficult to extract data or needed information.  There may also be some bias in papers and fact sheets from particular bodies with a financial interest in a field.

How to find grey literature?

Grey literature can be found from numerous sources, and isn't as easy as searching a database.  Depending on the research question, there will be more appropriate sources to use than others.  Use the grey literature sources box to get started.

What if you don't want to search grey literature?

According to McAuley, grey literature in meta-analyses has been shown to change the results of whether interventions are considered effective or not.

Grey Literature Sources

Clinical trial registries provide information about studies that are in process, and could affect the systematic review results once the study is concluded.  The concluded studies with results may also not be published, and will assist in reducing publication bias.

Conference abstracts and posters may contain relevant information, that may never be published as a full journal article.  The Agency for Healthcare Research and Quality recommend reviewers to use conference proceedings to identify those unpublished studies.  Some conference material may appear in database results from PubMed or Academic Search Complete.  However, a few other sources may also need to be reviewed.  Use the hand searching box for additional options.

Hand Searching

Hand searching involves browsing resources to find relevant studies.  Hand searching is used because not all journals are well indexed, and many resources are not indexed in a database and can only be browsed.  If time is limited, a well-developed search would retrieve the majority of English-language only study reports.

Hand searching for conference abstracts and posters, use:

  • Supplement issues of on topic journals
  • Topic appropriate association conference websites

Cited Reference Searching

The process involves reviewing reference lists of included articles and looking for highly relevant articles, such as related reviews.  It is important to review references to find articles that were not found in the search, either because they were not indexed in a database or did not use common keywords.

Sources for cited references include:

  • Browsing reference lists of articles
  • Using a citation tracking database like Google Scholar

Working with a Librarian

As a team member, the librarian can select or suggest which sources to use for the search, design the search strategy, search the grey literature, save/collect the search results and share with team members, draft the search method, and obtain copies of trial reports for review teams.

If the librarian is serving as an advisory role, assistance and education is provided on the following items.

  • Advising authors on which databases and other sources to search
  • Providing guidance on designing search strategies for the main bibliographic databases and/or grey literature
  • Educate team members on saving and collecting search results
  • Advising authors on how to run searches in other sources and how to download results
  • Educate team members on documenting the search