Assessing the Risk of Bias of Individual Studies in Systematic Reviews of Health Care Interventions

In: Methods Guide for Effectiveness and Comparative Effectiveness Reviews [Internet]. Rockville (MD): Agency for Healthcare Research and Quality (US); 2008.


This document updates the existing Agency for Healthcare Research and Quality (AHRQ) Evidence-based Practice Center (EPC) Methods Guide for Effectiveness and Comparative Effectiveness Reviews on assessing the risk of bias of individual studies. As with other AHRQ methodological guidance, our intent is to present standards that can be applied consistently across EPCs and topics, promote transparency in processes, and account for methodological changes in the systematic review process. These standards are based on available empirical evidence, theoretical principles, or workgroup consensus: as greater evidence accumulates in this methodological area, our standards will continue to evolve. When possible, our recommended standards offer flexibility to account for the wide range of AHRQ EPC review topics and included study designs.

Some EPC reviews may rely on an assessment of high risk of bias to serve as a threshold between included and excluded studies; in addition, EPC reviews use risk-of-bias assessments in grading the strength of the body of evidence. Assessment of risk of bias as unclear, high, medium, or low may also guide other steps in the review process, such as study inclusion for qualitative and quantitative synthesis, and interpretation of heterogeneous findings.

This guidance document begins by defining terms as appropriate for the EPC program, explores the potential overlap in various constructs used in different steps of the systematic review, and offers recommendations on the inclusion and exclusion of constructs that may apply to multiple steps of the systematic review process. We note that this guidance applies to reviews—such as AHRQ-funded reviews—that separately assess the risk of bias of outcomes from individual studies, the strength of the body of evidence, and applicability of the findings. This guidance applies to comparative effectiveness reviews that require interventions with comparators and systematic reviews that may include noncomparative studies. A key construct, however, is that risk-of-bias assessments judge whether the design and conduct of the study compromised the believability of the link between exposure and outcome. This guidance may not be relevant for reviews that combine evaluations of risk of bias or quality of individual studies with applicability.

Later sections of this guidance document provide guidance on the stages involved in assessing risk of bias and design-specific minimum criteria to evaluate risk of bias. We discuss and recommend tools and conclude with guidance on summarizing risk of bias.

Publication types

  • Review