Definition · Plain-language
Grey literature
Grey literature comprises research, reports, and documents produced and published by organisations outside traditional academic, commercial, or scholarly publishing channels.
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Defining the scope of grey literature
Grey literature spans a vast range of formats that escape traditional bibliographic control. According to the Prague Definition, it is produced on all levels of government, academics, business, and industry in electronic and print formats, where publishing is not the primary activity of the producing body. This includes institutional white papers, preprints, regulatory filings, clinical trial registries, and patents. These sources often contain highly detailed technical information that is omitted from condensed journal articles, making them valuable resources for researchers looking for detailed methodologies and raw experimental data that standard databases miss. This inclusion of non-journal resources allows researchers to build a comprehensive view of historical development and current trends.
The value of grey literature in systematic reviews
For systematic reviews and meta-analyses, grey literature is essential for combating publication bias. Academic journals tend to publish statistically significant, positive findings while rejecting null results. By searching grey literature — such as thesis databases, working paper repositories, and government archives — researchers can locate unpublished studies and null findings, providing a more balanced and accurate synthesis of the evidence. Including these documents prevents the overestimation of treatment effects, which is a common issue when meta-analyses rely exclusively on peer-reviewed journal articles, skewing empirical results. Thus, grey literature search represents a critical tool for researchers who are committed to presenting an unbiased state of current scientific evidence.
Evaluating and citing grey sources
Because grey literature is not subject to standard peer review, researchers must evaluate it with extra care. Assessing the credibility of the authoring organisation, the transparency of their methodology, and any potential conflicts of interest is critical. Tools like the AACODS checklist help structure this evaluation process. When citing grey literature, researchers should provide stable URLs, publication dates, and institutional authors, ensuring that readers can locate the source document. Proper citation supports research transparency and allows other investigators to access and verify the referenced reports and documents. This transparency allows future meta-analyses to incorporate these grey sources with the same degree of confidence as standard journal articles.
Key facts
At a glance
- Published by organisations whose primary activity is not commercial publishing
- Includes policy reports, government documents, working papers, and theses
- Crucial for reducing publication bias in systematic literature reviews
- Often provides highly detailed technical data omitted from short journal papers
- Lacks standard peer-review filters, requiring rigorous manual evaluation
- Searchable via institutional repositories, think tank portals, and Google Scholar
Common misconceptions
What people often get wrong
Often heard: Grey literature is of low quality and should not be used in academic research.
Actually: Many grey sources, such as government reports and academic working papers, are written by leading experts and contain valuable, high-quality empirical data.
Often heard: Grey literature is peer-reviewed in the same way as journal articles.
Actually: Grey literature is usually published directly by organisations without external double-blind peer review, making independent evaluation of their methods essential.
Common questions
FAQ
Why is it called grey literature?+
The term "grey" refers to the intermediate or ambiguous status of these publications, which lie in a grey area between formal, commercial publishing (white literature) and completely private, unpublished communications (black literature).
How do you search for grey literature systematically?+
Researchers search specialised databases like OpenGrey, ProQuest Dissertations, government agency websites, clinical trial registries (like ClinicalTrials.gov), and think tank archives, using pre-planned search strings.
Going deeper
