Grey Matters: An Introduction to the Grey Literature and Where to Find It

Within the methods section of many a systematic review, it is common to come across the term "grey literature." Put plainly, grey literature comprises pieces of evidence that are not formally published in a book or peer-reviewed journal article. 

Examples of grey literature that can be valuable to a systematic review include:

• conference abstracts and proceedings
• clinical study reports
• dissertations and theses
• journal preprints

Searching the "grey lit" has several important benefits:

• It expands the reach of a systematic review beyond the scope of the databases mined by a search, increasing the chance of finding pieces of evidence that may be helpful to the final synthesis of data.
• It helps reduce the impact of potential publication bias on the findings of a review.
• It keeps the review current by including upcoming data from recent conferences, doctoral work, and other yet-to-be-published sources.

Ideally, a search of the grey literature should be used in tandem with other forms of hand-searching, including the searching of relevant citations within included articles and of well-known reviews on similar topic.

Where to Find Grey Literature

Below are some resources that list helpful links for exploring the grey literature:

• Cochrane Handbook Online
• Duke University
• The University of Illinois at Chicago