Using Grey Literature in your Research: Introduction

“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• Preprints or other unpublished manuscripts
• Unpublished clinical trial reports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.

• More complete
  • Including grey literature provides a broader evidence base. Completeness is important for systematic reviews, and also for scoping reviews and state-of-the-evidence reviews [3], which aim for broad and up-to-date literature coverage respectively.
• 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.

• 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].