Cochrane handbook recommends specific forms of grey literature be searched for compliant systematic reviews in the health sciences.
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Grey literature is a term that dates back to the 1920s. The International Conference on Grey Literature (ICGL) provides the following definition that has been mostly accepted in academia:
Grey Literature “refers not to the physical appearance of a document but to the uncertain status of it.”
-- Auger CP, 1988, Information Sources in Grey Literature, 4th ed, London, Bowker-Saur.
"Grey literature is recognized as a key source of evidence, argument, innovation, and understanding in many disciplines including science, engineering, health, social sciences, education, the arts and humanities" --
Pisa Declaration on Policy Development for Grey Literature Resources
What differentiates traditional grey literature from published literature?
It’s key purpose is to overcome, or at least minimise, publication or reporting bias.
“…published trials tend to be larger and show an overall greater treatment effect than grey trials.”
(Hopewell et al. 2007, p.2).
¨Campbell reviews must include a systematic search for unpublished reports (to avoid publication bias)”
(Campbell Collaboration 2017)
Many scientifically valid studies, especially those with negative results may never be published. If your review is solely based on published literature, which tends to report positive results, your conclusions are more likely to exaggerate.
In a lot of cases that exaggeration can be significant.