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Using Grey Literature in your Research: Introduction

A guide to searching and using grey literature

What is grey literature?

“Grey literature is any document not issued by an entity with publishing as its primary purpose” [1].

 Examples of grey literature include:

  • Dissertations and theses
  • Conference abstracts or proceedings
  • Preprints or other unpublished manuscripts
  • Unpublished clinical trial reports
  • Reports issued by governments, industry, think tanks, or NGOs
  • Data sets
  • Speeches, blogs, and social media

Grey literature is especially important when conducting a systematic review, which aims to include ALL evidence relevant to the research question.

Grey literature can complement traditional literature [2], by:

  • Corroborating its findings, or
  • Illustrating gaps between research and practice, and/or
  • Challenging its assumptions.

Why use grey literature?

  • More complete
  • Less biased
    • Including grey literature can mitigate the effects of publication bias [4,5,6], because studies with null effects are less likely to be published in traditional journals. The degree to which grey literature reduces publication bias may vary by sub-fields and research questions [7].
  • More inclusive
    • Grey literature can be produced by people outside of academia, including educators, clinicians, and policy makers, who have different expertise than a typical academic [2,8].
  • More up-to-date
    • Grey literature may be released more quickly than traditional peer-reviewed literature [3]. 
  • More relevant
    • Including inputs from people outside of academia can increase the relevance of a review for non-academic audiences.

Challenges of using grey literature

  • Lack of clear guidelines
    • Development of search strategy and protocol require more planning and careful documentation to ensure reproducibility
  • Near-infinite amount of information
    • It's not possible to search the whole internet; decisions about where to search and how many results to screen need to be made on a review-by-review basis.
  • Poor indexing and search functionality of grey literature databases and search engines
    • There is often no indexing of key terms and no capacity for complex search strings
  • Lack of abstracts
    • Researcher must often read the full text to assess relevance
  • Lack of consensus about the inclusion of grey literature
    • In some fields, grey literature is not broadly accepted; lack of peer review leads to questions about its quality. However, publication status is often a bad proxy for quality [6,9]. A quality assessment step, already recommended for all literature included in systematic reviews [2], should help mitigate this concern.


Systematic Reviews aim to find, evaluate, and synthesize all evidence relevant to a particular research question. They follow a structured approach to ensure a comprehensive literature search and to minimize bias [10].

Scoping Reviews take a similar comprehensive, structured approach to systematic reviews, but instead of focusing on a particular research question, they aim to assess the nature of existing literature on a topic and uncover potential gaps.

State-of-the-evidence reviews include a broader range of evidence than systematic reviews, including non-research literature, and aim for the most up-to-date coverage [3]. 

1. Young S, Premji Z, Engelbert M. Unit 3: Searching the Literature. In: Valentine JC, Littell JH, Young S, editors. Systematic reviews and meta-analysis: A Campbell Collaboration online course. Open Learning Initiative, 2023. Available from

2. Adams RJ, Smart P, Huff AS. Shades of Grey: Guidelines for Working with the Grey Literature in Systematic Reviews for Management and Organizational Studies. Int J Manag Rev. 2016;19(4):432-54. [link]

3. Benzies KM, Premji S, Hayden KA, Serrett K. State-of-the-evidence reviews: advantages and challenges of including grey literature. Worldviews Evid Based Nurs. 2006;3(2):55-61. [link]*

4. Burdett S, Stewart LA, Tierney JF. Publication bias and meta-analyses: a practical example. Int J Technol Assess Health Care. 2003;19(1):129-34. [link]*

5. McAuley L, Pham B, Tugwell P, Moher D. Does the inclusion of grey literature influence estimates of intervention effectiveness reported in meta-analyses? Lancet. 2000;356:1228-31. [link]*

6. Wilson DB. Missing a critical piece of the pie: simple document search strategies inadequate for systematic reviews. J Exp Criminol. 2009;5(4):429-40. [link]*

7. Hartling L, Featherstone R, Nuspl M, Shave K, Dryden DM, Vandermeer B. 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 Med Res Methodol. 2017;17(1):64. [link]

8. Mahood Q, Van Eerd D, Irvin E. Searching for grey literature for systematic reviews: challenges and benefits. Res Synth Methods. 2014;5(3):221-34. [link]*

9. Conn VS, Valentine JC, Cooper HM, Rantz MJ. Grey literature in meta-analyses. Nurs Res. 2003;52(4):256-61. [link]*

10. Grant MJ, Booth A. A typology of reviews: an analysis of 14 review types and associated methodologies. Health Info Lib J. 2009;26:91-108. [link]*

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