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