Grey literature is generally material not published commercially or indexed by major databases.
A more complete definition is information "produced on all levels of government, academics, business and industry in electronic and print formats not controlled by commercial publishing i.e. where publishing is not the primary activity of the producing body." (http://www.greynet.org/)
It may include: Reports, preprints, working documents, research papers, theses and dissertations, clinical trial registrations, bibliographies, newsletters, patents, statistical documents, white papers, pamphlets, datasets, informal communication (e.g., blogs, podcasts, email), and more.
The most appropriate grey literature for your review will depend on your research topic. Not all of the possible sources listed below will apply. To develop the most relevant grey literature search strategy for your project, consider:
More about grey literature from the Cochrane Handbook for Systematic Reviews of Interventions
Why include grey literature?
Cochrane's Methodological Expectations for Cochrane Intervention Reviews standards recommends that authors: "Search relevant grey literature sources such as reports, dissertations, theses, databases and databases of conference abstracts. Searches for studies should be as extensive as possible in order to reduce the risk of publication bias and to identify as much relevant evidence as possible."
For other research projects, the grey literature may be:
The Agency for Healthcare Research and Quality describes the reasons and methods for including grey literature in "Finding Grey Literature Evidence and Assessing for Outcome and Analysis Reporting Biases When Comparing Medical Interventions: AHRQ and the Effective Health Care Program."
Appraising grey literature
As with journal articles included in reviews, grey literature must be appraised before incorporating it into a review.
Unpublished studies and clinical trials can be appraised using the same tools as published studies and trials.
For other types of grey literature, the AACODS checklist provides guidance for evaluating Authority, Accuracy, Coverage, Objectivity, Date, Significance in the material. The checklist was developed by Jess Tyndall, Medical Librarian and Head of the Gus Fraenkel Medical Library at Flinders University as an evaluation and Critical Appraisal Tool specifically for use with grey literature sources, and it can be applied to materials across formats and disciplines.
Remember to record your search strategy for your grey literature searches too!
A preprint is a version of a scholarly or scientific manuscript that has not gone through the formal peer review, editing, or journal-publishing process. While a manuscript might be only in preprint status due to poor research or writing, there are also reasons unrelated to quality. These include publication bias¹, submitting to journals based on reputation rather than subject matter match, and the time required for peer review.²
Researchers might choose to include preprints for topics that are under-studied, novel, or rapidly-changing. For example, over 30,000 preprints on COVID-19 were hosted on preprint servers between January 1, 2020 and October 31, 2020 as researchers raced to address the pandemic.³
Sources for preprints:
³Fraser N, Brierley L, Dey G, et al. The evolving role of preprints in the dissemination of COVID-19 research and their impact on the science communication landscape. PLoS Biol. 2021;19(4):e3000959. Published 2021 Apr 2. doi:10.1371/journal.pbio.3000959. PMID: 33798194.
An Institutional Repository is a virtual space where a university or research institute collects and preserves its research and findings. Information in Repositories is considered grey literature since these resources are not traditionally published.
Use the search engines below to search keywords across many IGO (inter-governmental organization) or NGO (non-governmental organization) websites. For example, a single search for "microfinance" using the Union of International Associations IGO search will find reports and research on this topic from USAID, World Bank, Asian Development Bank, and many more.
When using the NGO search or other custom Google search, you can apply the methods from the Google Searching section to create complex queries. Google does not offer the same kind of control as the subscription databases do, but these tips can help you build searches that approximately correspond with your database searches.
The Deep Web, also known as the Invisible Web, is a portion of the web not reached by standard search engines, such as Google and Bing. Less than 10% of the web is indexed by search engines, with the remaining 90% of web content called the Deep Web. It is estimated to be 2-500x bigger than the visible web.
Here are some general resources to search the Deep Web:
Some of the health sciences databases include types of grey literature. Here are a few: