Table 2

Concepts in the assessment of risk of bias

Domain-based risk of bias assessmentsDomain-based risk of bias assessments identify specific study limitations that can introduce different biases (eg, bias arising from the randomisation process or bias due to deviation from intended interventions). Specific risk of bias domains provide insight into why a study outcome might be distorted, and by how much. For example, RCTs at ‘high’ risk of bias due to inadequate allocation concealment will be associated, on average, with overestimated trial outcomes in favour of the experimental group compared with RCTs at ‘low’ risk of bias.5
Assessing review outcomes separatelyStudy limitations that inform judgements of ‘some concerns’ or ‘high’ risk of bias can distort outcome measures differently.5 For example, pain is more likely to be overestimated when a patient is aware of their allocation to a specific intervention group (due to lack of patient blinding) than if they were not aware of their group allocation.5 22 Conversely, a patient’s awareness of their allocation to an intervention group is less likely to influence an outcome such as reinjury.5 22 Systematic review authors should perform separate risk of bias assessments for each outcome rather than assessing all review outcomes at once with one, general risk of bias assessment. A domain-based risk of bias assessment for separate outcomes evaluates the judgements of each risk of bias domain for separate review outcome types.