This section describes the recommended approach for assessing risk of bias in studies included in Cochrane reviews. It is a two-part tool, addressing the six specific domains discussed in Sections 8.9 to 8.14 (namely sequence generation, allocation concealment, blinding, incomplete outcome data, selective outcome reporting and ‘other issues’). The tool is summarized in Table 8.5.a. Each domain includes one or more specific entries in a ‘Risk of bias’ table. Within each entry, the first part of the tool involves describing what was reported to have happened in the study. The second part of the tool involves assigning a judgement relating to the risk of bias for that entry. This is achieved by answering a pre-specified question about the adequacy of the study in relation to the entry, such that a judgement of ‘Yes’ indicates low risk of bias, ‘No’ indicates high risk of bias, and ‘Unclear’ indicates unclear or unknown risk of bias.
The domains of sequence generation, allocation concealment and selective outcome reporting should each be addressed in the tool by a single entry for each study. For blinding and for incomplete outcome data, two or more entries may be used because assessments generally need to be made separately for different outcomes (or for the same outcome at different time points). Review authors should try to limit the number of entries used by grouping outcomes, for example, as ‘subjective, or ‘objective’ outcomes for the purposes of assessing blinding; or as ‘patient-reported at 6 months’ or ‘patient-reported at 12 months’ for incomplete outcome data. The same groupings of outcomes will be applied to every study in the review. The final domain (‘other sources of bias’) can be assessed as a single entry for studies as a whole (the default in RevMan). It is recommended, however, that multiple, pre-specified, entries be used to address specific other risks of bias. Such author-specified entries may be for studies as a whole or for individual (or grouped) outcomes within every study. Adding new entries involves specifying a question that should be answerable as ‘Yes’ to indicate a low risk of bias.