Grey literature
Grey literature is a type of scientific information that is not formally published as articles in scholarly journals. It could be reports, dissertations, manuscripts, clinical guidelines, produced by governments, universities and private companies.
Why search and use grey literature?
Grey literature can be an important complement because it allows you to find information that is not available elsewhere. Grey literature can provide different perspectives, and can be used to highlight and discuss a topic in a broader context. Grey material can also provide a more detailed description of a particular method or technique, for example.
You may also find more up-to-date information in grey literature compared to other sources. Grey literature usually does not undergo the same peer review as scientific publications, and therefore does not take as long to publish. It may also include information that has not yet been accepted for publication in a scientific journal. Because the grey literature does not follow the same standardized reporting structure as scientifically published articles, its quality and reliability may vary.
Grey literature in systematic reviews
Searching for and including grey literature in systematic reviews is mentioned and recommended in several different handbooks and checklists, including the Cochrane Handbook and PRISMA. It is also common to include grey literature in other types of reviews, such as scoping reviews.
One reason for including grey literature in systematic reviews is to reduce the risk of publication bias. Publication bias means that studies with positive results are more likely to be published than studies with negative results, which can lead to an overrepresentation of studies with positive results in the published scientific literature.
Searching for grey literature
To find grey literature, you may need to use several different approaches and sources. Searching in Google Scholar or other search engines can be a good start, but to avoid missing relevant literature, other approaches are also recommended. How many and which sources to use depends on the amount of time and resources you have available - searching for grey literature can be time-consuming - and on the type of grey material you want to find.
In general, you need to use a simplified search strategy when searching for grey literature compared to searching in regular databases. In some sources, you need to search with only one or a few keywords.
Many sources where you find grey literature have limited possibilities when it comes to exporting the results to, for example, a reference management program. You may therefore need to make the selection right during the search process.
If you want to learn more
Summarized Research in Information Retrieval for HTA (SuRe Info) has an informative and recently updated chapter on grey literature.
Searching the Literature: A Guide to Comprehensive Searching in the Health Sciences - Grey literature detailed guide from the University of Toronto Libraries. Includes not only tips on resources to search, but also suggested approaches and documentation.
Where to search for grey literature
Below are some resources and search services that can be useful for finding different types of grey material.
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