Table 1 A comparison of Cochrane risk of bias approaches since 2006

2006

Recommendation was made to assess selection bias (allocation concealment), performance bias, attrition bias and detection bias for every study.

Selection bias (specifically allocation concealment) was incorporated into Review Manager and scored as ‘adequate (A), unclear (B), inadequate (C) or that allocation concealment was not used (D)’. Other biases were not mandatory and likely not consistently assessed. No justification of judgement was required.

2008

Development of a tool for assessing risk of bias. Four biases as above, with ‘other bias’ category for authors to consider additional biases. No limit on the number of additional biases authors can identify.

Sequence generation, allocation concealment, blinding, incomplete outcome data, selective outcome reporting and other domains which authors could add.

Assessment of six domains within five biases mandatory. Judgement of ‘Yes’ (low risk), ‘No’ (high risk) or ‘Unclear’. Justification of judgement mandatory for each domain for each study.

2011

2008 tool modified but no new biases added.

As per 2008 version, except blinding divided into two domains, one related to blinding of participants and personnel, and the other related to outcome assessors.

Judgements changed to ‘High risk’, ‘Low risk’ or 'Unclear risk’. Clarification of which category of bias domain refers to and additional guidance for other risk of bias.