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8.5.1  Overview

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 seven specific domains discussed in Sections 8.9 to 8.15 (namely sequence generation, allocation concealment, blinding of participants and personnel, blinding of outcome assessment, incomplete outcome data, selective outcome reporting and ‘other issues’). The tool is summarized in Table 8.5.a. Note that the tool was revised in late 2010 after an evaluation project. Changes made at that point are summarized in Table 8.5.b.

Each domain in the tool includes one or more specific entries in a ‘Risk of bias’ table. Within each entry, the first part of the tool describes what was reported to have happened in the study, in sufficient detail to support a judgement about the risk of bias. The second part of the tool assigns a judgement relating to the risk of bias for that entry. This is achieved by assigning a judgement of ‘Low risk’ of bias, ‘High risk’ of bias, or ‘Unclear 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 of participants and personnel, blinding of outcome assessment 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 of outcome assessment; 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 bias’) can be assessed as a single entry for studies as a whole (the default in RevMan). However, it is strongly recommended that 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.