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Grey Literature and Other Sources: Types

An overview of key search strategies to locate grey literature, informal or unpublished material.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

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Types of grey literature

While some types of grey literature are commonly included in systematic searching, such as conference abstracts and theses, to some extent the type of grey literature you need depends on your question and the evidence required to answer it.

For example, randomised controlled trials are typically used for questions focused on the effectiveness of specific interventions. For this type of question it would be important to include trials registers in your sources.

If instead you were investigating user perceptions of a particular service, a qualitative design incorporating interviews or a questionnaire might be more appropriate. The study might never have been published, but mentioned in a report available via the organisation's website.

You should carefully consider the types of grey literature you will and will not include.

The following video summarises some common types.

Video Length: 5:13


  • It is important to look outside of the published literature by searching for grey literature.
  • Including grey literature in your research will help minimise publication or reporting bias toward positive results.
  • Grey literature is '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 body of activity of the producing body' (ICGL Luxembourg definition 1997).
  • It can include:
    • blogs
    • clinical trials and practice guidelines
    • conference papers
    • government documents
    • patient handouts
    • policy statements
    • reports
    • standards
    • statistics
    • theses
    • videos
    • websites
  • Searching grey literature can be useful when your research field has very little published evidence.
  • Grey literature can be found through Library databases, Google Scholar, internet search engines and specific websites.
  • Locating grey literature can sometimes be time consuming, frustrating, and may not always result in useful evidence.
conference abstracts conference papers patents statistics
blogs emails preprints theses and dissertations
brochures curriculum policy statements translations
software government documents reports videos
clinical trial register entries informal communications repository content webpages
practice guidelines archival material standards wiki articles

Explore further:

Guidelines and standards

These sections of the Cochrane Handbook recommend specific types of grey literature to be searched for compliant systematic reviews.

Hierarchy of evidence

Hierarchy of evidence pyramid. From the bottom of the pyramid to the top: studies, synopses of studies, syntheses, synopses of syntheses, summaries, and systems. As you move up the pyramid the level of evidence increases but the amount of studies published decreases.

Adapted from DiCenso, Bayley & Haynes 2009, p. 100. 

For more about the hierarchy, visit the Evidence-Based Practice guide.