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Systematic reviews and other evidence synthesis projects

What is grey literature?

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." (

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:

  • Who would likely have written about your topic?
    • Government agencies?
    • Non-governmental organizations?
    • Academic institutions?
    • Industry?
  • ​What kinds of literature would help answer your research question?
    • Clinical trials?
    • Theses and dissertations?
    • Conference proceedings?
    • Reports?
    • Statistics?
  • Which time periods or geographic areas are relevant to your research?

More about grey literature from the Cochrane Handbook for Systematic Reviews of Interventions

Why include grey literature?

The intent of a systematic review is to synthesize all available evidence that is applicable to your research question. 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."

Material in the grey literature may be:

  • a better source of information on policies and programs
  • trials indicating no effect or negative results, which are less likely to be published than positive results
  • a source of different perspectives than major publications offer
  • more current, with better coverage of emergent research areas
  • more detailed than journal articles, with raw data or more extensive context

The Agency for Healthcare Research and Quality describes 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."

Remember to record your search strategy for your grey literature searches too!

  • Identify and record the sources you will search. The sources you search will be informed by your research question and where you expect to find information related to your question.
  • Document where you are searching and your search strategies, including document resource name, URL, search terms, and date searched.
  • Collect citation information as you go.
  • Adhere to your established inclusion and exclusion criteria when selecting sources.

Clinical Trials

  • CenterWatch: Global source for clinical trials information, offering news, analysis, study grants, career opportunities, and trial listings to professionals and patients
  • Registry and results database of federally and privately supported clinical trials conducted in the United States and around the world
  • EU Clinical Trials Register: Registered clinical trials in the European Union 
  • ISRCTN Register: Registered randomized controlled trials and other studies to assess the efficacy of health care interventions around the world 
  •  ICTRP: The WHO International Clinical Trials Registry Platform searches in and the ISRCTN Register along with 16 other international clinical trials registries.

Reports and Research


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:

¹Guyatt GH, Oxman AD, Montori V, et al. GRADE guidelines: 5. Rating the quality of evidence--publication bias. J Clin Epidemiol. 2011;64(12):1277-1282. doi:10.1016/j.jclinepi.2011.01.011. PMID: 21802904.
²Penfold NC, Polka JK. Technical and social issues influencing the adoption of preprints in the life sciences. PLoS Genet. 2020;16(4):e1008565. Published 2020 Apr 20. doi:10.1371/journal.pgen.1008565. PMID: 32310942.

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.

Dissertations & Theses at UW and beyond

Other materials

Institutional Repositories

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.

  • ResearchWorks at the University of Washington: digital repository of articles, technical reports, dissertations, datasets, images and other file types produced at UW by faculty and researchers.
  • OpenDOAR: directory of open access repositories

IGO/NGO Search Engines

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 these searches or other custom Google searches, 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.

Tip: At first it will appear that you have thousands of results. However, if you page through them there will actually be only a few pages.

Deep Web

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:

Grey literature in the databases

Some of the health sciences databases include types of grey literature. Here are a few:

  • PubMed: some preprints
  • Embase: conference proceedings and abstracts; preprints
  • CINAHL: dissertations; articles from industry news and magazines
  • PsycInfo: dissertations; conference proceedings
  • Web of Science: conference proceedings and abstracts; articles from industry news and magazines