To establish transparency of systematic review results and findings, a Risk of Bias Assessment is often performed for each included study. Systematic reviews strive to eliminate bias in their findings. Individual studies that are included in a systematic review may include biases in their results or conclusions, for example design flaws that raise questions about validity of findings or an overestimate of intervention effect. Risk of bias assessment generally is not required with evidence synthesis methods outside of systematic reviews. However, this may depend on the evidence synthesis method that you are utilizing.
Many tools have been developed to assess risk of bias. However, for systematic reviews in human medicine, the Cochrane Collaboration warns against using scales that yield a summary score, and instead advises authors to individually assess each study using a simple judgment of low risk, high risk or unclear risk on different axes. These assessments can be represented in table format in a systematic review, for example*:
If a high proportion of studies are assessed with a high risk of bias, caution should be used when interpreting results for your systematic review. More information can be found in Chapter 8 of the Cochrane Handbook.
This Cochrane Training presentation helps to navigate the steps of Risk of Bias Assessment, but some things might not apply to other disciplines. For disciplines outside of human medicine, the Critical Appraisal Skills Programme provides checklists that can be applied to a diverse array of study types.