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Grey Literature and Other Sources: Overview

An overview of key search strategies to locate grey literature, informal or unpublished material. Provides sources and instructions on reporting requirements for systematic reviews.

Overview title banner

What and why of grey literature

'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'

- 1997 International Conference on Grey Literature, Luxembourg - definition expanded in New York, 2004 

A few distinguishing characteristics of grey literature - typically...

  • limited distribution
  • difficult to find
  • not formally part of 'traditional' publishing models
  • highly variable in quality
  • may not have undergone peer review
  • ongoing availability not guaranteed
  • file formats may vary

This Guide focuses on 'other' sources and approaches - everything other than searching standard 'bibliographic' databases typically dominated by journal articles, such as MEDLINE, PsycInfo and Business Source Complete. The relevant material you find may be 'grey' or published.

'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, 2014

Grey literature:

  • introduces alternative perspectives
  • may be the only source of information (where there is little published evidence)
  • overcomes or minimises publication/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 published research and fills in the research gaps
  • can contain more local information
  • helps offset the bias in published results e.g. drug trials

Grey literature as a part of 'other methods' in PRISMA 2020 ('other sources' in PRISMA 2009)

Excerpt from the PRISMA 2020 flow diagram

PRISMA 2020 flow diagram template for systematic reviews - excerpt [Page et al. (2021). The PRISMA 2020 statement: an updated guideline for reporting systematic reviews. BMJ, 372, n71.]

A range of approaches and sources are recommended to ensure that you locate all relevant studies (as part of an 'exhaustive' search). 

For example, the Cochrane Manual recommends contacting 'relevant individuals and organizations', searching trials registers, and backward and forward citation searching, alongside searching for grey literature such as reports and conference abstracts. This is everything other than the core search of databases that primarily index published literature such as journal articles.

In terms of reporting, PRISMA 2020 indicates that records from databases and study registers be recorded in one section, and records identified via 'other methods' in a separate box to the right. 

Guidelines and standards

The Cochrane Handbook recommends specific forms of grey literature (which it defines as 'reports [of studies] published outside of traditional commercial publishing') be searched for compliant systematic reviews in the health sciences. It also recommends a number of alternative approaches to locating relevant studies.

Visit the handbook for more information.

PRISMA 2020 details reporting requirements for information sources and strategies.

Grey literature in evidence based practice

Its 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)

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 regarding effects are more likely to be exaggerated.

Further reading