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Systematic Reviews: Searching the Gray Literature

A Guide to Conducting Systematic Reviews

How to 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.

Google Scholar

Google Scholar has it's place in research. That being said, GS is not great for doing a systematic review. The filters are not good, you can't build an advance search, and the Google algorithm is unknown. It can be used for gray literature searching through. Do a simplified search in GS and use the first 300 results or the first few pages of results. 

The gray literature: What is it and how do I search it?

What is gray literature? 

Gray literature is defined by the Cochrane Handbook for Systematic Reviews of Interventions as "...literature that is not formally published in sources such as books or journal articles." This can include information such as government reports, conference proceedings, graduate dissertations, unpublished clinical trials, and much more.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 gray literature if it hasn't been peer-reviewed? 

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  Meanwhile, many studies and trials that show no effect end up going unpublished.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.  Thus, the gray literature can be critical.

How do I search the gray literature if it's unpublished and not in the scholarly databases?  

Its true--finding gray 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:

  • Identify and record the sources you will search, including some indication of search terms used if appropriate.  The sources you search will be informed by your research question and where you expect to find information related to your question.
  • Try searching databases that specialize in gray literature like OpenGrey.  See the box below for more information.
  • Conference proceedings:  Identify professional organizations that have conferences at which researchers might be presenting work related to your topic.  Search those conference proceedings on the organization's website or by contacting organizational boards for access to past proceedings that may not be online.
  • Theses and dissertations:  There are a number of databases dedicated to theses and dissertations, which you can search using your search terms.  See the box below 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.  For example, the WHO has a number of searchable online collections and the World Bank now makes all of their publications openly available online.
  • Contact known researchers in the field to determine if there are any ongoing or unpublished studies that s/he may be aware of.

Grey Literature Sources

Googling the Greys: Tips for Searching Beyond Health Databases and Turning Information into Insights

A thorough grey literature search should involve a general sweep of the web by using different search engines. Google is an important search engine but other search engines, such as Yahoo and Bing, could also be useful.

While Google is a powerful tool for searching for grey literature it should not be used exclusively - other sources should also be searched in order to find grey literature. 

Some tips:

1) Restrict content to .org or .gov sites

Type in your topic and then either "" OR ""

2) Restrict content to file type

Type in your topic and then "filetype:pdf" OR "filetype:doc"

3) Use Google Australia, Google UK, etc.

4) Use the Duck Duck Go search engine which does not record location or user searches

5) Use Link Klipper (Chrome extension) to pull results into a Microsoft Excel spreadsheet. You can also use the Grey Literature Search Log form linked below to keep track of your searches:

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