A Guide to Evidence Synthesis: 3. Select Grey Literature Sources

What is Grey Literature? 

Grey (or gray) literature is literature produced by individuals or organizations outside of commercial and/or academic publishers. This can include information such as government reports, conference proceedings, graduate dissertations, unpublished clinical trials, and much more.  The sources you select will be informed by your research question and field of study, but should likely include, at a minimum, theses and dissertations.

Why Search the Grey Literature? 

The intent of an evidence synthesis is to synthesize all available evidence that is applicable to your research question. There is a strong bias in scientific publishing toward publishing studies that show some sort of significant effect.  Meanwhile, many studies and trials that show no effect end up going unpublished.  But knowing that an intervention had no effect is just as important as knowing that it did have an effect when it comes to making decisions for practice and policy-making. While not peer-reviewed, grey literature represents a valuable body of information that is critical to consider when synthesizing and evaluating all available evidence.

How Do I Search the Grey Literature?  

Finding grey literature and searching it systematically is challenging.  But there are a few approaches that you can take to add some structure to your search of this type of information:

• Refer to grey literature sources used for related evidence syntheses: Refer to both published evidence syntheses and registered protocols. 
• Ask experts in the field for relevant grey literature sources: If you are an expert, include important grey literature sources, and ask colleagues for their recommendations.
• Search databases that specialize in grey literature: See the "Grey Literature Sources" box at the bottom of this page for more information.
• Search for theses and dissertations:  There are a number of databases dedicated to theses and dissertations, which you can search using your search terms. See the "Grey Literature Sources" box at the bottom of this page for links to these resources. 
• Search clinical trials: There may be clinical trials being conducted that are relevant to your research question, but that haven't been published yet or never were published. See the "Grey Literature Sources" box at the bottom of this page for links to these resources. 
• Identify government agencies and international and non-governmental organizations that might publish technical papers and reports on your topic.  Search their websites or any online libraries that they may provide.  See the "Grey Literature Sources" box at the bottom of this page for links to some examples.
• Search conference proceedings and newsletters: Identify professional organizations that have and/or conferences at which researchers might be presenting work related to your topic. Search those conference proceedings or newsletters on the organization's website or by contacting organizational boards for access to past proceedings that may not be online. See the "Grey Literature Sources" box at the bottom of this page for some examples.
• Contact known researchers in the field to determine if there are any ongoing or unpublished studies that s/he may be aware of.
• Search professional and trade magazines: Professional magazines contain literature that is written by professionals in the field for other professionals in the field, but that may not be about research. Trade magazines contain advertisements and news very specific to a topic or industry.

How Do I Manage the Grey Literature Search?

• 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.

See below for guidance documents specific to grey literature searching.

Grey Literature Resources