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"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 Meanwhile, many studies and trials that show no effect end up going unpublished.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 While not peer-reviewed, gray literature represents a valuable body of information that is critical to consider when synthesizing and evaluating all available evidence."
Farrah, K., & Mierzwinski-Urban, M. (2019). Almost half of references in reports on new and emerging nondrug health technologies are grey literature. Journal of the Medical Library Association : JMLA, 107(1), 43–48. https://doi.org/10.5195/jmla.2019.539