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Systematic Reviews: the process: Grey Literature

About Grey Literature

The Twelfth International Conference on Grey Literature in Prague in 2010 arrived at the following definition:

"Grey literature stands for manifold document types produced on all levels of government, academics, business and industry in print and electronic formats that are protected by intellectual property rights, of sufficient quality to be collected and preserved by libraries and institutional repositories, but not controlled by commercial publishers; i.e. where publishing is not the primary activity of the producing body."

Examples of grey literature include: conference abstracts, presentations, proceedings; regulatory data; unpublished trial data; government publications; reports (such as white papers, working papers, internal documentation); dissertations/theses; patents;  and policies &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

Inclusion of grey literature into a systematic review is recommended in order to help minimize publication bias. The inclusion of grey literature in systematic reviews is widely recognized as important and international organizations have incorporated this information in their guidelines and manuals for working on reviews and meta-analyses.

  • AHRQ: Finding Grey Literature Evidence and Assessing for Outcome and Analysis Reporting Biases When Comparing Medical Interventions
    This is a chapter from AHRQ's "Methods Guide for Effectiveness and Comparative Effectiveness Reviews."
  • Cochrane Handbook: Unpublished and Onging Studies
    From the Cochrane Handbook for Systematic Reviews of Interventions. Version 5.1.0 [updated March 2011]
  • Cochrane Handbook: Including unpublished studies in systematic reviews
    From the Cochrane Handbook for Systematic Reviews of Interventions. Version 5.1.0 [updated March 2011]
  • The Joanna Briggs Institute Reviewer's Manual
    Aromataris E, Munn Z (Editors). Joanna Briggs Institute Reviewer's Manual. The Joanna Briggs Institute, 2017. Available from
  • PRISMA for systematic review protocols (PRISMA-P)
    Item 9: "Describe all intended information sources (such as electronic databases, contact with study authors, trial registers or other grey literature sources) with planned dates of coverage."

Searching the Grey Literature

Searching the grey literature can be a daunting task. You should search those resources that make the most sense for your research question. At a minimum, consider searching Abstracts and Conferences. If your question involves drugs and interventions, check trial registries and pharma data. Also check out the papers and reports of relevant stakeholder organizations.

The links below may shed some additional light on the process.

Sources of Grey Literature

The approval process for new drugs and devices involves submission of data that might not be published elsewhere. These documents can help identify publication bias even when complete methodological details of unpublished trials are unavailable. This information is not available prior to a drug’s approval and may be heavily redacted; however, when available, reviewers can compare results of published and unpublished trials.

If further information is requested, these are additional resources to consider:

Online trial registries may include results of completed but unpublished clinical trials. These resources can be helpful in identifying otherwise unreachable trials and provide additional details of published trials. Many individual drug companies also have posted trial registries on their websites, although their quality varies. Even without results, knowledge that the trial exists can be helpful for reviewers, because they can contact the principal investigator for more information. 

If further information is requested, these are additional resources to consider: