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Copy of Systematic Reviews RD May 24 to consult with Academic Skills: Grey Literature

Search strategy design: Selecting sources guidance




"A range of resources are searched where possible: searching only one database is not sufficient for a systematic review as no one database covers all reports of research. The choice of databases is influenced by the research question, but generally, as a minimum for reviews of clinical topics, databases should include MEDLINE, EMBASE and CENTRAL.

The search must seek to identify all relevant reports, including those classed as grey literature (material that is not formally published, such as institutional or technical reports, technology assessments, conference proceedings, or other documents that are not normally subject to editorial control or peer review). Grey literature can be searched using specialised search engines, databases or websites. Additional methods used to identify studies include hand-searching selected journals or conference proceedings, seeking expert opinion, searching the reference lists of included papers or relevant reviews, carrying out citation searches on included studies, and searching clinical trials registers to identify ongoing and recently completed research. Seeking as much available literature as possible minimises the potential for publication bias."

McCool, R., & Glanville, Julie. (2014). What is a systematic review? Evidence Based Health care

Guidance from Cochrane handbook


See 4.S1 Technical Supplement to Chapter 4: Searching for and selecting studies Section 1.1.6 Grey literature databases:


Publication bias and systematic reviews

"The search for, and inclusion of, grey literature in a systematic review is an important way to help overcome some of the problems of publication bias"

Hopewell, S., Clarke, M., & Mallett, S. (2005). Grey literature and systematic reviews. Publication bias in meta-analysis: Prevention, assessment and adjustments, 48-72.

Guidance from Centre for Reviews & Dissemination, University of York

In addition to searching electronic databases, the Centre for Reviews and Dissemination at University of York recommend locating studies by:

 • Visually scanning reference lists from relevant studies

 • Handsearching key journals and conference proceedings

 • Contacting study authors, experts, manufacturers, and other organisations

 • Searching relevant Internet resources

Internet searching can be a useful means of retrieving grey literature, such as unpublished papers, reports and conference abstracts. Identifying and scanning specifi c relevant websites will usually be more practical than using a general search engine such as ‘Google’. Reviews of transport and ‘welfare to work’ programmes have reported how Internet searching of potentially relevant websites was effective in identifying additional studies to those retrieved from databases.40, 41 It is worth considering using the Internet when investigating a topic area where it is likely that studies have been published informally rather than in a journal indexed in a bibliographic database. Internet searching should be carried out in as structured a way as possible and the procedure documented (see Appendix 3).

 • Citation searching  See also

Citation searching involves selecting a number of key papers already identified for inclusion in the review and then searching for articles that have cited these papers. This approach should identify a cluster of related, and therefore highly relevant, papers

 Citation searching used to be limited to using Web of Science and Scopus, but other resources (including CINAHL, PsycINFO, and Google Scholar) now include cited references in their records so these are also available for citation searching. Using similar services offered by journals such as the BMJ can also be helpful.


• Using a project Internet site to canvas for studies

 University of York. NHS Centre for Reviews & Dissemination. Systematic Reviews : CRD's Guidance for Undertaking Reviews in Healthcare. York: Centre for Reviews & Dissemination, 2009.

Grey literature and systematic reviews

Mahood Q.Van Eerd D. and Irvin E. (2014), Searching for grey literature for systematic reviews: challenges and benefitsResearch Synthesis Methods5,(3): 221234, doi: 10.1002/jrsm.1106

Haddaway NR, Collins AM, Coughlin D, Kirk S (2015) The Role of Google Scholar in Evidence Reviews and Its Applicability to Grey Literature Searching. PLoS ONE 10(9): e0138237. doi: 10.1371/journal.pone.0138237

Saleh, A. A., Ratajeski, M. A., & Bertolet, M. (2014). Grey literature searching for health sciences systematic reviews: A prospective study of time spent and resources utilized. Evidence Based Library and Information Practice, 9(3), 28-50. DOI:

Searching for Grey Literature

Some of our databases, depending on the discipline, index grey literature as well as journals.

Check the database scope note.


See also the following sources:


Google Scholar

Lenus : the Irish Health Repository



40 million publications, 1 million research data sets

from 17,000 content providers and 22 funders linked together for an integrated research search.


Registry of Open Access Repositories (ROAR)

Social Science Research Network

Social Science Research Network (SSRN) is devoted to the rapid worldwide dissemination of social science research and is composed of a number of specialized research networks in each of the social sciences.


A global library catalogue which  combines the catalogues of over 20,000 libraries from over 100 different countries.


Trials Registers


EU Clinical Trials Register

UK Clinical Trials Gateway

Development of a single international trials register at ClinicalTrials.Gov

ICTRP International Clinical Trials Registry Platform

See Medical and health-related trials registers and research registers 

This resource provides a listing of trials and research registers and a quick reference guide to the search basics for each resource. If you would like to suggest registers to add to this resource please send the details to Julie Glanville at

In addition see Cochrane Handbook Chapter Six: 6.2.3 Unpublished and ongoing studies:

National and international trials registers

Subject-specific trials registers

Pharmaceutical industry trials registers

Trials results registers and other sources

See also:

Cochrane Handbook Chapter Ten:  10.3.2  Including unpublished studies in systematic reviews:

 Including unpublished studies in systematic reviews

Higgins, Julian., Green, Sally  and Cochrane Collaboration. Cochrane Handbook for Systematic Reviews of Interventions. Chichester, England: Wiley-Blackwell, 2011 


Conference Proceedings

Theses & Dissertations

See our Theses and dissertations webpage.

See also the full list of national and international theses databases to which we provide access.