Grey Literature: Overview

Conform with the Health Sciences standard

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:

"Information produced on all levels of government, academics, business and industry in electronic and print formats not controlled by commercial publishing, i.e. where publishing is not the primary activity of the producing body" 
-ICGL Luxembourg definition, 1997-expanded in New York, 2004
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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.

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"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

  • Introduces alternative perspectives.Stop sign [Image source: Pixabay, https://pixabay.com/en/stop-drawing-icon-comics-road-sign-1207069/, copied under CC0 1.0 Public domain dedication, https://creativecommons.org/publicdomain/zero/1.0/deed.en]
  • May be the only source of information where there is little published evidence.
  • Overcomes, or minimises publication or reporting bias.
  • Can be essential in emerging, changing or interdisciplinary research areas e.g. alternative medicine.
  • May find unique information not found elsewhere, especially in emerging research areas.
  • Complements research or fills in the research gaps.
  • Can contain more local information.
  • Can use it to generate an independent reference set to check your search is on track.
  • Helps offset the bias in published results e.g. drug trials.

What differentiates traditional grey literature from published literature?

  • Not widely disseminated.
  • Difficult to find.
  • Often not archived.
  • Not formally part of the “traditional publishing” models.
  • Speed of production especially with self publishing on the internet.
  • Lower costs than traditional publishing.
  • Highly variable quality, lack of established peer review processes.
  • Archived material frequently in formats and/or hardware/software to access the content may also be obsolete

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Grey literature in Evidence Based Practice

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.

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Further reading

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