Skip to main content
It looks like you're using Internet Explorer 11 or older. This website works best with modern browsers such as the latest versions of Chrome, Firefox, Safari, and Edge. If you continue with this browser, you may see unexpected results.

Systematic Reviews: Grey literature

A guide for researchers undertaking a systematic review

Finding 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  • unpublished or ongoing studies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.


Hopewell  S, McDonald  S, Clarke  MJ, Egger  M. Grey literature in meta‚Äźanalyses of randomized trials of health care interventions. Cochrane Database of Systematic Reviews 2007, Issue 2. Art. No.: MR000010. DOI: 10.1002/14651858.MR000010.pub3. Available from: 
Lefebvre C, Manheimer E, Glanville J. Searching for studies. In: Higgins JPT, Green S, editors. Cochrane handbook for systematic reviews of interventions. Chichester: Wiley-Blackwell; 2008.

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). For additional sources in your subject area please contact your Faculty librarian.



  • 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 



  • 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
  • OpenDOAR, a directory of academic open access repositories


  • Australian Institute of Health and Welfare, a comprehensive source for Australian health reports
  • BASE, a multidisciplinary search engine to scholarly internet resources created by the Bielefeld University Library in Germany. Finds articles, conference papers, reports etc

  • Campbell Collaboration Library, maintains and disseminates systematic reviews in education, disability, nutrition, social welfare, and crime and justice
  • CORE. a collection of open access research papers

  • 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
  • OpenGrey, (formerly SIGLE), covers science, technology, biomedical science, economics, social science and humanities grey literature
  • The Grey Literature Report, a bimonthly publication of The NY Academy of Medicine Library, highlighting new grey literature publications in health services research and selected public health topics
  • WHOLIS (World Health Organization library database) includes international material from a variety of sources
  • Mednar, a deep web search engine that conducts real-time searching of peer-reviewed medical resources
  • WorldWideScience, a global science gateway comprised of national and international scientific databases and portals

Trial registries

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.

For more information about searching Google see our guide on Using Google and Google Scholar.



This video, produced by Western University, Canada, provides an overview of grey literature and how it might be used by researchers:

Evaluating grey literature

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