Gray Literature (Grey Literature) is literature that is not published through traditional means and is often excluded from traditional academic databases. Grey literature is:
Source: http://www.greylit.org/about
Examples of Gray Literature include, but are not limited to,
Sources of Gray Literature may be located in commercial databases, websites and/or university repositories.
"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 Meanwhile, many studies and trials that show no effect end up going unpublished.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 While not peer-reviewed, gray literature represents a valuable body of information that is critical to consider when synthesizing and evaluating all available evidence."
Source: https://guides.library.cornell.edu/evidence-synthesis/gray-literature
