In the presence of confounding, the association between intervention and outcome differs from its causal effect. Bias in selection of participants into the study The study appears to provide sound evidence for a non-randomized study but cannot be considered comparable to a well-performed randomized trial. A revised tool to assess risk of bias in randomized trials (RoB 2) Welcome to the website for the RoB 2 tool. Using big data to emulate a target trial when a randomized trial is not available. Unclear = not enough information to make a clear judgement • support for judgement •direct quotes from the paper or study author where possible •additional comments

Such a study will usually be judged to be at serious or critical risk of bias because it is impossible to determine whether pre-post changes are due to the intervention rather than other factors. Other sources of bias.

The Cochrane Handbook for Systematic Reviews of Interventions is the official guide that describes in detail the process of preparing and maintaining Cochrane systematic reviews on the effects of healthcare interventions.All authors should consult the Handbook for guidance on the methods used in Cochrane systematic reviews.The Handbook … The only post-baseline deviation that may lead to bias are the potentially biased actions of researchers arising from the experimental context. If follow-up time is re-allocated to the alternative intervention in the analysis that produced the result being assessed for risk of bias, then there is a potential for bias arising from time-varying confounding.

Bias that arises when later follow-up is missing for individuals initially included and followed (e.g. Hernán MA, Hernandez-Diaz S, Werler MM, Mitchell AA. Evaluating risk of bias in results of NRSI requires both methodological and content expertise.

Criteria for the judgement of ‘High risk’ of bias. * For the precise wording of signalling questions and guidance for answering each one, see the full ROBINS-I tool at The tool and a guidance on how to use it can be found here. Assessment of risk of bias may, for some domains, rely heavily on expert opinion rather than empirical data: this means that consensus may not be reached among experts with different opinions. ‘Moderate’ risk of bias in multiple domains may lead review authors to decide on an overall judgement of ‘Serious’ risk of bias for that outcome or group of outcomes, and ‘Serious’ risk of bias in multiple domains may lead review authors to decide on an overall judgement of ‘Critical’ risk of bias. There may be only one unit, several units or many units. * For the precise wording of signalling questions and guidance for answering each one, see the full ROBINS-I tool at

If particular questions/entries were pre-specified in the review’s protocol, responses should be provided for each question/entry. Table 2 2 presents an example of a risk of bias table for one trial included in a Cochrane review of therapeutic monitoring of antiretrovirals for people with HIV.

Adjusting for factors that are Assessment of bias in this domain will depend on the effect of interest (either the effect of assignment to intervention or the effect of adhering to intervention). The study has one or more important problems.

Bias in selection of participants into the study Our use of the term ‘selection bias’ is intended to refer only to bias that would arise even if the effect of interest were null, that is, biases that are internal to the study, and not to issues of indirectness (generalizability, applicability or transferability to people who were excluded from the study) (Schünemann et al 2013). The latest version (22 August 2019) is suitable for individually-randomized, parallel-group trials. Other bias. In some studies measurements of the outcome variable are made both before and after an intervention takes place. A revised Cochrane risk of bias tool for randomized trials. For example, there is potential for bias when prevalent users of an intervention (those already receiving the intervention), rather than incident (new) users are included in analyses comparing them with non-users. Selection bias can also occur when some follow-up time is excluded from the analysis. Cochrane Reviews often include non-randomized studies of interventions (NRSI), as discussed in detail in