Version 2 of the Cochrane risk-of-bias tool for randomized trials (RoB 2) is the recommended tool to assess the risk of bias in randomized trials included in Cochrane Reviews. RoB 2 is structured into a fixed set of domains of bias, focussing on different aspects of trial design, conduct, and reporting. Within each domain, a series of questions ('signalling questions') aim to elicit information about features of the trial that are relevant to risk of bias. A proposed judgement about the risk of bias arising from each domain is generated by an algorithm, based on answers to the signalling questions. Judgement can be 'Low' or 'High' risk of bias, or can express 'Some concerns'. The tool and guidance on how to use it can be found here.
Version 2 of the tool replaces the first version, originally published in Version 5 of the Cochrane Handbook in 2008, and updated in 2011 (see here). Research in the field has progression and RoB 2 reflects current understanding of how the causes of bias can influence study results, and the most appropriate ways to assess this risk.