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Objective: To describe the bias assessment practice in recently published systematic reviews of mediation studies and to evaluate the quality of different bias assessment tools for mediation analysis proposed in the literature. Method: We conducted an overview of systematic reviews by searching MEDLINE (OvidSP), PsycINFO (OvidSP), Cochrane Database of Systematic Reviews (OvidSP), and PubMed databases for systematic reviews of mediation studies published from 2007 to 2020. Two reviewers independently screened the title, abstracts, and full texts of the identified reports and extracted the data. The publications of all mediation-specific quality assessment tools used in these reviews were also identified for the evaluation of the tools’ development and validation. Result: Among 103 eligible reviews, 24 (23%) reviews did not assess the risk of bias of eligible studies, and 48 (47%) assessed risk of bias using a tool that was not specifically designed to evaluate mediation analysis. 31 (30.1%) reviews assessed the risk of mediation-specific biases, either narratively or by using specific tools for mediation studies. However, none of these tools were consensus-based, rigorously developed or validated. Conclusion: The quality assessment practice in recently published systematic reviews of mediation studies is suboptimal. To improve the quality and consistency of risk of bias assessments for mediation studies, a consensus-based bias assessment tool is needed.

Original publication




Journal article


Journal of clinical epidemiology

Publication Date





137 - 148