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Systematic Reviews: Grey Literature

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What is grey literature

“There are many definitions of grey literature, but it is usually taken to mean literature that is not formally published in sources such as books or journal articles.” Cochrane Handbook-  Grey literature databases.

Formats include:

  • Registered Controlled Trial Registers
  • Technical or research reports from government agencies
  • Reports from scientific research groups
  • Working papers from research groups or committees
  • Doctoral (PhD) dissertations
  • Some conference proceedings and official publications
  • Preprints (journal articles not yet peer-reviewed and/or published)

Why search the Grey Literature?

The intent of a systematic review is to synthesize all available evidence that is applicable to your research question. There is a strong bias in scientific publishing toward publishing studies that show some sort of significant effect.  Meanwhile, many studies and trials that show no effect end up going unpublished.  But knowing that an intervention had no effect is just as important as knowing that it did have an effect when it comes to making decisions for practice and policy-making.  While not peer-reviewed, grey literature represents a valuable body of information that is critical to consider when synthesizing and evaluating all available evidence.

Grey Literature Guide

Controlled trial registers

Trial Registers are a useful source of unpublished and ongoing trials:

Preprint Repositories

Preprint is a term that tends to mean a version of a manuscript that is self-archived and shared publicly before publication in a scholarly journal - often the version prior to peer review. You may wish to include searches for preprints in your systematic review.

There are a number of different sources for finding preprints. For example:

  • MedRxiv: launched in June 2019, this health sciences preprint server is a collaborative effort between Cold Spring Harbor Laboratory, BMJ and Yale. Submissions are screened to deter potential risk to public health.
  • bioRxiv: launched in 2013, this repository contains research from the life sciences and is maintained by Cold Spring Harbor Laboratory. bioRxiv facilitates direct transfer of submissions to participating journals.
  • OSF preprints: several partner repositories are hosted on the Open Science Framework (OSF) preprint platform. The partner repositories vary by discipline, region and screening process.
  • See a comprehensive list of preprint servers at:
  • See also this paper for useful information on finding preprints: Langham-Putrow, A., & Riegelman, A. (2019). Discovery and scholarly communication aspects of preprints: Sources for online information. College & Research Libraries News, 80(9), 506-510.

Reporting grey literature searching

When you are writing up your review remember to report on any grey literature searching you have done.

Describe any online or print source purposefully searched or browsed (e.g., tables of contents, print conference proceedings, web sites) and how this was done.

  • “We also searched the grey literature using the search string: "public attitudes" AND "sharing" AND "health data" on Google (in June 2017). The first 20 results were selected and screened.”
  • “The grey literature search was conducted in October 2015 and included targeted, iterative hand searching of 22 government and/or research organization websites that were suggested during the expert consultation and are listed in S1 Protocol. Twenty two additional citations were added to the review from the grey literature search.”
  • “To locate unpublished studies, we searched Embase [via] for conference proceedings since 2000 and hand-searched meeting abstracts of the Canadian Conference on Physician Health and the International Conference on Physician Health (2012 to 2016).”

Read more: PRISMA-S: An Extension to the PRISMA Statement for Reporting Literature Searches in Systematic Reviews

Grey Literature Resources