RoB 2

Assessing risk of bias in the results of individual randomized controlled trials (RCTs) included in a systematic review or evidence synthesis. RoB 2 is the recommended risk-of-bias tool for randomized trials in Cochrane Reviews.

• Parallel-group randomized controlled trials (the primary version of RoB 2)
• Cluster-randomized trials (using the dedicated RoB 2 cluster-randomized extension, not currently available in CoRATES)
• Crossover trials (using the dedicated RoB 2 crossover extension, not currently available in CoRATES)

RoB 2 organises the assessment around five bias domains for parallel-group randomized trials. For each domain, reviewers answer signalling questions whose responses are mapped, via published algorithms, to a domain-level judgement of Low risk of bias, Some concerns, or High risk of bias. The five parallel-group domains are listed below. Cluster-randomized and crossover extensions add additional domains specific to those designs; consult the official documentation linked under Reference Documents for the extension-specific signalling questions and guidance.

1. 1.Bias arising from the randomization process

   Considers whether the allocation sequence was adequately random and concealed until participants were enrolled and assigned to interventions, and whether the baseline characteristics of the groups suggest a problem with the randomization process.

2. 2.Bias due to deviations from intended interventions

   Considers whether participants and trial personnel were aware of the assigned intervention, whether deviations from the intended intervention occurred and were balanced between groups, and whether the appropriate analysis was used to estimate the effect of assignment to or adherence to the intervention. RoB 2 explicitly distinguishes between the effect of assignment and the effect of adhering to the intervention.

3. 3.Bias due to missing outcome data

   Considers the proportion of participants with missing outcome data, whether missingness was likely to depend on the true value of the outcome, and whether the missing data could plausibly have biased the result of the trial.

4. 4.Bias in measurement of the outcome

   Considers whether the outcome measurement methods were appropriate, comparable across intervention groups, and unlikely to be influenced by knowledge of the intervention received. The signalling questions distinguish subjective and objective outcomes because the former are more vulnerable to detection bias.

5. 5.Bias in selection of the reported result

   Considers whether the reported effect estimate was selected, on the basis of the result, from among multiple eligible outcome measurements or analyses of the data. This domain captures selective non-reporting and outcome switching.

Use RoB 2 when you are assessing risk of bias in individual randomized trials as part of a systematic review or evidence synthesis. Each RoB 2 assessment should be performed for a specific result of interest (a particular outcome and effect estimate at a specific time point), not for the trial as a whole. Reviewers should pre-specify whether they are assessing the effect of assignment to the intervention or the effect of adhering to the intervention, because the signalling questions and algorithms differ. For Cochrane systematic reviews of randomized trials, RoB 2 is the recommended tool. For non-Cochrane reviews, RoB 2 is also widely used and is generally preferred over the original Cochrane RoB tool.

When not to use it

RoB 2 is not designed for non-randomized studies of interventions; for those, use ROBINS-I V2. It is also not the right tool for assessing the methodological quality of a systematic review (use AMSTAR 2 for that), for diagnostic test accuracy studies, or for prognostic factor studies. The parallel-group version of RoB 2 should not be applied to cluster-randomized or crossover trials without using the corresponding extensions, because those designs introduce additional bias domains that the parallel-group version does not assess.

RoB 2 sits alongside two other Cochrane risk-of-bias tools: ROBINS-I V2 (for non-randomized studies of interventions) and ROBINS-E (for non-randomized studies of exposures, published in 2024). All three tools share a common structure of signalling questions feeding domain-level and overall judgements, but they differ in the specific bias domains they assess and the algorithms used to derive judgements. For systematic reviews that include both randomized and non-randomized studies of the same intervention, it is appropriate to apply RoB 2 to the trials and ROBINS-I V2 to the non-randomized studies, then synthesise the results with appropriate consideration of the differing risk-of-bias structures. RoB 2 replaces the original 2008 Cochrane RoB tool, which is now considered outdated; new Cochrane Reviews are expected to use RoB 2 unless there is a strong reason to use the older tool.

• RoB 2 vs ROBINS-I: which risk-of-bias tool for your study?
• RoB 1 vs RoB 2: what changed in the Cochrane risk-of-bias tool

CoRATES presents RoB 2 as a structured digital workflow rather than a spreadsheet or PDF form. Each signalling question is rendered in context with its response options, reviewers can attach rationale and direct quotes from the trial report to each judgement, and the domain-level and overall judgements are computed automatically from responses using the official algorithms. Multiple reviewers can complete independent assessments of the same trial result in parallel; CoRATES then surfaces the disagreements in a side-by-side reconciliation view so reviewers can resolve them before locking the assessment. Completed assessments can be exported as publication-ready risk-of-bias visual summaries (traffic-light plots and weighted bar plots).

• RoB 2 official tool and templates (riskofbias.info)
• Cochrane Bias Methods Group: RoB 2 overview
• Sterne et al. (2019): RoB 2 publication, BMJ
• Cochrane Handbook Chapter 8: Assessing risk of bias in included studies

The original Cochrane Risk of Bias tool was introduced in version 5 of the Cochrane Handbook in 2008 (Higgins et al.) and was updated in 2011. RoB 2 (Sterne et al., BMJ 2019) is a substantive replacement rather than a minor revision. Notable structural changes from the original tool include: (1) the assessment is now structured around fixed bias domains with explicit signalling questions, rather than free-form judgements; (2) algorithms map signalling-question responses to suggested domain-level risk-of-bias judgements; (3) the intermediate "Some concerns" judgement was added between Low and High; (4) RoB 2 is applied per result (per outcome and analysis), not once per trial; (5) the tool explicitly distinguishes the effect of assignment from the effect of adhering to the intervention; and (6) the deviations and missing outcome data domains were substantially expanded. Extensions of RoB 2 for cluster-randomized trials and crossover trials are available and add design-specific domains.

• Assessing the trial as a whole rather than a specific result. RoB 2 is intended to be applied per result (per outcome and analysis); applying it once to the entire trial can mask domain-specific risks that only affect particular outcomes.
• Failing to specify whether you are assessing the effect of assignment or the effect of adhering to the intervention. The signalling questions and algorithms differ between the two, and conflating them produces ambiguous judgements.
• Using parallel-group RoB 2 on cluster-randomized or crossover trials. Each of these designs has its own RoB 2 extension that adds domains specific to the design; the parallel-group version omits those design-specific bias sources.
• Confusing RoB 2 with the original 2008 Cochrane RoB tool. The two tools produce different judgements and have different domain structures; reviews using the older tool should not be combined with RoB 2 assessments without explicit acknowledgement of the difference.
• Treating "Some concerns" as a default for ambiguous cases. "Some concerns" should be a substantive judgement, not a way to avoid choosing between Low and High when the available information would actually support a definitive judgement.
• Overriding the algorithm-suggested judgement without recording rationale. The algorithms are an important consistency aid; deviations should be documented so reviewers and downstream readers can audit the reasoning.
• Single-reviewer assessment. Like all risk-of-bias appraisals, RoB 2 should be carried out by at least two independent reviewers with disagreements resolved through discussion or arbitration.

What does RoB 2 stand for?

RoB 2 stands for Risk of Bias 2, the second version of the Cochrane Risk of Bias tool for randomized trials. The "2" distinguishes it from the original 2008 Cochrane Risk of Bias tool, which RoB 2 replaces.

How is RoB 2 different from the original Cochrane Risk of Bias tool?

RoB 2 introduced a fixed set of bias domains with structured signalling questions, replacing the more free-form judgements of the original tool. It added algorithms that map signalling-question responses to suggested domain-level judgements, introduced the intermediate "Some concerns" rating between Low and High, applies the assessment per result (per outcome and analysis) rather than once per trial, and explicitly distinguishes between the effect of assignment to the intervention and the effect of adhering to the intervention. The domains for deviations from intended interventions and missing outcome data were also substantially expanded.

How many domains does RoB 2 assess?

The parallel-group version of RoB 2 assesses five bias domains: bias arising from the randomization process, bias due to deviations from intended interventions, bias due to missing outcome data, bias in measurement of the outcome, and bias in selection of the reported result. The cluster-randomized and crossover extensions of RoB 2 add additional domains specific to those designs.

Should I use RoB 2 or ROBINS-I V2?

Use RoB 2 for randomized trials and ROBINS-I V2 for non-randomized cohort studies of interventions. If your systematic review includes both designs, apply each tool to the appropriate subset of studies and synthesise the results with awareness of the differing risk-of-bias structures.

Is RoB 2 the recommended tool for randomized trials in Cochrane Reviews?

Yes. Cochrane has formally adopted RoB 2 as the recommended tool for assessing risk of bias in randomized trials included in Cochrane Reviews. Reviews still using the original 2008 tool are gradually being updated to RoB 2.

Are there extensions of RoB 2 for other trial designs?

Yes. There are dedicated RoB 2 extensions for cluster-randomized trials and crossover trials, both of which add design-specific bias domains beyond the five domains of the parallel-group version. The extensions are hosted alongside the main tool at riskofbias.info. CoRATES currently implements the parallel-group version only.

Does CoRATES automatically score RoB 2 assessments?

CoRATES applies the published RoB 2 algorithms to map signalling-question responses to domain-level and overall risk-of-bias judgements, and updates them in real time as reviewers answer questions. CoRATES does not modify the official scoring algorithms.

Where are the official RoB 2 signalling questions and guidance?

The official RoB 2 instrument, signalling questions, and guidance documents are hosted at riskofbias.info, and the original publication is freely available in the BMJ (Sterne et al., BMJ 2019;366:l4898). CoRATES does not reproduce the official content; reviewers should consult the official sources alongside their assessments. See Reference Documents above for direct links.

Members of the RoB 2 Development Group, including Julian P.T. Higgins, Jelena Savovic, Matthew J. Page, Roy G. Elbers, and Jonathan A.C. Sterne, with contributions from many additional collaborators across the Cochrane Bias Methods Group. The tool was published in the BMJ in 2019 (Sterne et al., BMJ 2019;366:l4898).

• Sterne JAC, Savovic J, Page MJ, et al. (2019). RoB 2: a revised tool for assessing risk of bias in randomised trials. BMJ 2019;366:l4898. View
• Higgins JPT, Altman DG, Gotzsche PC, et al. (2011). The Cochrane Collaboration's tool for assessing risk of bias in randomised trials. BMJ 2011;343:d5928. View