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Systematic Reviews & Other Syntheses


What is grey literature?

Grey literature is the term used to describe literature that is not formally published as a book or journal article (Higgins, 2011), including:

  • conference abstracts and papers
  • preprints
  • dissertations and theses
  • governmental or private sector research
  • ongoing or unpublished clinical trial data
  • statistical publications
  • internal reports or working papers
  • technical reports

Related library guides:

In addition to the grey literature resources listed below, there are related library guides on: Finding Theses and Dissertations and Government Information (Health). The Public Health library guide has a page on grey literature with additional sources to search as well.

Grey Literature Resources

Some library databases contain grey literature in addition to journal articles. For example, Web of Science and Embase contain conference proceedings, while PsycINFO contains some dissertations. Since library databases do not generally provide comprehensive coverage of grey literature, other resources should be searched to identify literature that is not formally published as well.

Preprint Servers

Clinical Trial Registries

For quantitative systematic reviews of healthcare interventions, it is strongly recommended to search clinical trial registries for on-going and unpublished trials in order to limit publication bias. 

Note: Since August 2019, eligible reports of RCTs or quasi-RCTs from and WHO ICTRP are being identified and systematically added to Cochrane CENTRAL (other study designs registered (e.g. cohort studies) are not included).


Baudard, M., Yavchitz, A., Ravaud, P., Perrodeau, E., & Boutron, I. (2017). Impact of searching clinical trial registries in systematic reviews of pharmaceutical treatments: methodological systematic review and reanalysis of meta-analyses. BMJ, 356.

Godin, K., Stapleton, J., Kirkpatrick, S. I., Hanning, R. M., & Leatherdale, S. T. (2015). Applying systematic review search methods to the grey literature: a case study examining guidelines for school-based breakfast programs in Canada. Systematic Reviews, 4(1). 

Haddaway, N. R., Collins, A. M., Coughlin, D., & Kirk, S. (2015). The role of Google Scholar in evidence reviews and its applicability to grey literature searching. PLoS ONE, 10(9). 

Hartling, L., Featherstone, R., Nuspl, M., Shave, K., Dryden, D. M., & Vandermeer, B. (2017). Grey literature in systematic reviews: a cross-sectional study of the contribution of non-English reports, unpublished studies and dissertations to the results of meta-analyses in child-relevant reviews. BMC Medical Research Methodology, 17(64), 1–11.

Higgins, J.P.T. & Green, S. (Eds.). (2011). 6.2.3  Unpublished and ongoing studies In Cochrane Handbook for Systematic Reviews of Interventions Version 5.1.0 [updated March 2011]. The Cochrane Collaboration. Available from

Hopewell, S., Mcdonald, S., Mj, C., & Egger, M. (2007). Grey literature in meta-analyses of randomized trials of health care interventions (Review). Cochrane Database of Systematic Rev, (2. Art. No.:MR000010.).

Mahood, Q., Van Eerd, D., & Irvin, E. (2014). Searching for grey literature for systematic reviews: challenges and benefits. Research Synthesis Methods, 5(3). 

McAuley, L., Pham, B., Tugwell, P., & Moher, D. (2000). Does the inclusion of grey literature influence estimates of intervention effectiveness reported in meta-analyses? The Lancet, 356(9237), 1228–1231. 

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. 

Schmucker, C. M., Blümle, A., Schell, L. K., Schwarzer, G., Oeller, P., Cabrera, L., … Consortium,  on behalf of the O. (2017). Systematic review finds that study data not published in full text articles have unclear impact on meta-analyses results in medical research. PLOS ONE, 12(4), 

Stansfield, C., Dickson, K., & Bangpan, M. (2016). Exploring issues in the conduct of website searching and other online sources for systematic reviews: how can we be systematic? Systematic Reviews, 5(1). 

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