Systematic review guide

The New York Academy of Medicine defines grey literature as “that which is produced on all levels of government, academics, business and industry in print and electronic formats, but which is not controlled by commercial publishers." Basically, grey literature is anything that is not a published article - such as a conference paper, health report or unpublished clinical trial. Ideally, you should include grey literature to ensure that you have used all the data currently available to guide your results. It also helps your review avoid publication bias.

See the following video for advice on including grey literature in your review.

There is no systematic way of searching for grey literature, the best way is to use a number of different sources, databases, repositories, websites and search engines. For some common sources of grey literature see the ‘Grey Literature Sources’ section of this guide. The Cochrane Handbook says that ClinicalTrials.Gov and the WHO International Clinical Trials Registry Platform (ICTRP) are the most important trial registers to search. You may also want to search for preprints. These are scientific manuscripts that have been uploaded to a public server for others to read for free. Initially they are posted without peer-review but they may get feedback or reviews and may eventually be published in a peer-reviewed journal. A quick way to do this is by searching Europe PMC as it covers the main biomedical preprint servers. In addition to this, consider possible organisations and websites that might include useful data, you could try looking for sources in the references of articles your database search has found and ask your supervisor, or experts in the field, for recommendations.

You can also try carrying out a simple search for grey literature on Google by including filetype:pdf or filetype:doc as part of your search. This means that your results will only include pdfs or Word documents. Do not rely on popular documents that are high up in the search results. Important documents can be easily retrieved via Google but some grey literature may be hidden within the results, down several pages or not visible at all due to a relative lack of popularity.