Systematic Reviews: Grey literature

What is grey literature?

The term grey literature "is usually understood to mean literature that is not formally published in sources such as books or journal articles" (Lefebvre, Manheimer, & Glanville, 2008, p. 106). Grey literature is a key source of evidence and argument in many subject areas and can include:

• conference papers/proceedings
• theses and dissertations
• clinical trials
• newsletters
• government documents
• unpublished or ongoing studies
• informal communications (blogs, emails, etc)

Why is grey literature important?

Grey literature helps you to form a more complete view of all the evidence around a particular topic. It is particularly important to include as part of your systematic review in order to limit bias due to publication lag and the publication of only positive results.

A systematic review conducted in 2008 by members of the Cochrane methodologies team found that often the results from grey literature significantly affect the outcome of a review, as they often report more negative or inconclusive data than published journal articles (Hopewell et al., 2008). As such, it is important to treat grey literature as another potential source of studies for inclusion.

Searching for grey literature

• Keep it simple! Return back to your core concepts, don't try to run long strings of search terms.
• Choose core resources and organisations.
• Familiarise yourself with how similar systematic reviews have approached their grey literature search.
• Record where, when and how you searched each resource.

Grey literature is usually not subject to peer review and should be evaluated accordingly. To assist with evaluation you may wish to refer to the AACODS checklist created by Jess Tyndall, Flinders University, covering:

• Authority
• Accuracy
• Coverage
• Objectivity
• Date
• Significance

There are a number of sources available to search for grey literature. Access to most grey literature is free (or can be freely accessed via library databases).

Catalogues

• Trove, a database of content and holdings from Australian libraries, museums, archives and research organisations. Includes reports, working papers, theses, conference proceedings, and unpublished material
• WorldCat, the world's largest network of library content and holdings, including theses and dissertations 

Repositories

• Institutional repositories, including Australasian Open Access Repositories, provides access to digital theses and research papers
• OAIster, provides access to worldwide digital theses, technical reports and research papers

• Campbell Collaboration Library, maintains and disseminates systematic reviews in education, disability, nutrition, social welfare, and crime and justice
• CogPrints, an electronic archive for self-archive papers in psychology, neuroscience, linguistics, and many areas of computer science, philosophy and biology
• Grey Matters, a practical tool and checklist for searching health-related grey literature
• IDEAS on RePEc, an economics database providing access to working papers and other research

To effectively search for grey literature using Google we recommend using Google Advanced Search.

Here you can enter your search terms and apply limits in the 'Narrow your results' section. For example:

Site or domain

• Consider restricting to a site or domain, though you can only search one domain name at a time.
• .edu or .ac for academic institutions
• .org for organisations
• .gov for government sites.

 File type

• Most of the grey literature available on the internet is in the form of PDF documents.
• Select 'Adobe Acrobat PDF' to limit to pdf results.

For more information about refining web searches see Google Search Help.