Risk of Bias Assessment (RoB 2) Explained

RoB 2 (the Cochrane risk-of-bias tool for randomized trials, second version) is the standard tool for judging how much confidence to place in a randomized trial's results. It doesn't ask whether a study is "good" in a general sense — it asks whether specific features of how the trial was conducted could have distorted the result.

The five domains

• Bias arising from the randomization process — was allocation genuinely random and concealed?
• Bias due to deviations from intended interventions — did participants or personnel know their group assignment in ways that could affect behavior or care?
• Bias due to missing outcome data — were dropouts and missing data handled appropriately, and were they related to the outcome?
• Bias in measurement of the outcome — could how the outcome was measured or assessed differ systematically between groups?
• Bias in selection of the reported result — was the reported result selected from among multiple analyses or outcome measures?

How judgments are made

Each domain is assessed using a set of signalling questions, leading to a domain-level judgment of "low risk," "some concerns," or "high risk." The overall study judgment is generally driven by the worst-rated domain — a trial can't be rated low risk overall if any single domain shows high risk of bias.

Why this matters for your review

Risk-of-bias results directly inform how much weight a study's findings should carry in your synthesis, and feed into GRADE certainty-of-evidence ratings at the outcome level. A synthesis that ignores risk of bias treats a rigorous trial and a deeply flawed one as equally trustworthy — which undermines the whole point of a systematic review.

RoB 2 vs. other tools

RoB 2 is specifically for randomized trials. Non-randomized studies need a different tool (commonly ROBINS-I), and observational designs like cohort or case-control studies often use the Newcastle-Ottawa Scale. Using the wrong tool for your study design is a common and avoidable error.