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RoB 2: a revised tool for assessing risk of bias in randomised trials

Jonathan A C Sterne (http://orcid.org/0000-0001-8496-6053),1 2 professor, Jelena Savović,1 3 senior research fellow, Matthew J Page,4 research fellow, Roy G Elbers,1 senior research associate, Natalie S Blencowe,1 2 Isabelle Boutron,5 6 7 professor, Christopher J Cates,8 senior clinical research fellow, Hung-Yuan Cheng,1 2 Mark S Corbett,9 research fellow, Sandra M Eldridge,10 professor, Jonathan R Emberson,11 associate professor, Miguel A Hernán,12 professor, Sally Hopewell,13 associate professor, Asbjørn Hróbjartsson,14 15 16 professor, Daniela R Junqueira,17 research associate, Peter Jüni,18 professor, Jamie J Kirkham,19 professor, Toby Lasserson,20 senior editor, Tianjing Li,21 associate professor, Alexandra McAleenan,1 senior research associate, Barnaby C Reeves,2 22 professorial research fellow, Sasha Shepperd,23 professor, Ian Shrier,24 investigator, Lesley A Stewart,9 professor, Kate Tilling,1 2 professor, Ian R White,25 professor, Penny F Whiting,1 3 senior lecturer, Julian P T Higgins,1 2 3 professor 1Population Health Sciences, Bristol Medical School, University of Bristol, Bristol BS8 2BN, UK2NIHR Bristol Biomedical Research Centre, Bristol, UK3NIHR CLAHRC West, University Hospitals Bristol NHS Foundation Trust, Bristol, UK4School of Public Health and Preventive Medicine, Monash University, Melbourne, Australia5METHODS team, Epidemiology and Biostatistics Centre, INSERM UMR 1153, Paris, France6Paris Descartes University, Paris, France7Cochrane France, Paris, France8Population Health Research Institute, St George’s, University of London, London, UK9Centre for Reviews and Dissemination, University of York, York, UK10Pragmatic Clinical Trials Unit, Centre for Primary Care and Public Health, Queen Marys University of London, UK11MRC Population Heath Research Unit, Clinical Trial Service Unit and Epidemiological Studies Unit, Nuffield Department of Population Health, University of Oxford, Oxford, UK12Departments of Epidemiology and Biostatistics, Harvard T H Chan School of Public Health, Harvard-MIT Division of Health Sciences of Technology, Boston, MA, USA13Centre for Statistics in Medicine, Nuffield Department of Orthopaedics, Rheumatology and Musculoskeletal Sciences, University of Oxford, Oxford, UK14Centre for Evidence-Based Medicine Odense, Odense University Hospital, Odense, Denmark15Department of Clinical Research, University of Southern Denmark, Odense, Denmark16Open Patient data Explorative Network, Odense University Hospital, Odense, Denmark17Department of Emergency Medicine, Faculty of Medicine and Dentistry, University of Alberta, Edmonton, Alberta, Canada

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18Applied Health Research Centre, Li Ka Shing Knowledge Institute, St Michael’s Hospital, Department of Medicine and Institute of Health Policy, Management and Evaluation, University of Toronto, Toronto, Ontario, Canada19Centre for Biostatistics, University of Manchester, Manchester, UK20Editorial and Methods Department, Cochrane Central Executive, London, UK21Department of Epidemiology, Johns Hopkins Bloomberg School of Public Health, Baltimore, Maryland, USA22Translational Health Sciences, Bristol Medical School, University of Bristol, Bristol, UK23Nuffield Department of Population Health, University of Oxford, Oxford, UK24Centre for Clinical Epidemiology, Lady Davis Institute, Jewish General Hospital, McGill University, Montreal, Quebec, Canada25MRC Clinical Trials Unit, University College London, London, UK

Correspondence to: J A C Sterne [email protected] (or @jonathanasterne on Twitter)Accepted: 25 June 2019

Assessment of risk of bias is regarded as an essential component of a systematic review on the effects of an intervention. The most commonly used tool for randomised trials is the Cochrane risk-of-bias tool. We updated the tool to respond to developments in understanding how bias arises in randomised trials, and to address user feedback on and limitations of the original tool.

An evaluation of the risk of bias in each study included in a systematic review documents

potential flaws in the evidence summarised and contributes to the certainty in the overall

evidence.1 The Cochrane tool for assessing risk of bias in randomised trials (RoB tool)2 has

been widely used in both Cochrane and other systematic reviews, with over 40 000 citations

in Google Scholar.

Many innovative characteristics of the original RoB tool have been widely accepted. It

replaced the notion of assessing study quality with that of assessing risk of bias (we define

bias as a systematic deviation from the effect of intervention that would be observed in a

large randomised trial without any flaws). Quality is not well defined and can include study

characteristics (such as performing a sample size calculation) that are not inherently related to

bias in the study’s results. The RoB tool considers biases arising at different stages of a trial

(known as bias domains), which were chosen on the basis of both empirical evidence and

theoretical considerations. Assessments of risk of bias are supported by quotes from sources

describing the trial (eg, trial protocol, registration record, results report) or by justifications

written by the assessor.

After nearly a decade of experience of using the RoB tool, potential improvements have

been identified. A formal evaluation found some bias domains to be confusing at times, with

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assessment of bias due to incomplete outcome data and selective reporting of outcomes

causing particular difficulties, and confusion over whether studies that were not blinded

should automatically be considered to be at high risk of bias.3 More guidance on

incorporating risk-of-bias assessments into meta-analyses and review conclusions is also

needed.4 5 A review of comments and user practice found that both Cochrane and non-

Cochrane systematic reviews often implemented the RoB tool in non-standard ways.6 Few

trials are assessed as at low risk of bias, and judgments of unclear risk of bias are common.6 7

Empirical studies have found only moderate reliability of risk-of-bias judgments.8

We developed a revised risk-of-bias assessment tool to address these issues, incorporate

advances in assessment of risk of bias used in other recently developed tools9 10 and to

integrate recent developments in estimation of intervention effects from randomised trials.11

Box startSummary pointsAssessment of risk of bias is regarded as an essential component of a systematic review on

the effects of an intervention; the most commonly used tool for assessing risk of bias in randomised trials is the Cochrane risk-of-bias tool, which was introduced in 2008

Potential improvements to the Cochrane risk-of-bias tool were identified on the basis of reviews of the literature, user experience and feedback, approaches used in other risk-of-bias tools, and recent developments in estimation of intervention effects from randomised trials

We developed and piloted a revised tool for assessing risk of bias in randomised trials (RoB 2)

Bias is assessed in five distinct domains. Within each domain users of RoB 2 answer one or more signalling questions. These answers lead to judgments of “low risk of bias”, “some concerns”, or “high risk of bias”

The judgments within each domain lead to an overall risk-of-bias judgment for the result being assessed, which should enable users of RoB 2 to stratify meta-analyses according to risk of bias

Box end

Development of the revised RoB toolWe followed the principles adopted for the development of the original RoB tool and for

the ROBINS-I tool for assessing risk of bias in non-randomised studies of interventions.2 9 A

core group coordinated development of the tool, including recruitment of collaborators,

preparation and revision of documents, and administrative support.

Preliminary work included a review of how the original tool was used in practice,6 a

systematic review and meta-analysis of meta-epidemiological studies of empirical evidence

for biases associated with characteristics of randomised trials,12 and a cross sectional study of

how selective outcome reporting was assessed in Cochrane reviews.13 We also drew on a

systematic review of the theoretical and conceptual literature on types of bias in

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epidemiology, which sought papers and textbooks presenting classifications or definitions of

biases, and organised these into a coherent framework (paper in preparation).

The core group developed an initial proposal and presented it, together with the latest

empirical evidence of biases in randomised trials, at a meeting in August 2015 attended by 24

contributors. Meeting participants agreed on the methodological principles underpinning the

new tool and the bias domains to be addressed, and formed working groups for each domain.

The groups were tasked with developing signalling questions (reasonably factual questions

with yes/no answers that inform risk-of-bias judgments), together with guidance for

answering these questions and broad considerations for how to judge the risk of bias for the

domain.

The materials prepared by the working groups were assembled and edited by the core

team, and the resulting draft was piloted by experienced and novice systematic reviewers

during a three day event in February 2016, with 17 participants present and 10 participants

contributing remotely. Issues identified in the pilot were recorded and addressed in a new

draft discussed at a second development meeting in April 2016, also attended by 24

contributors. Subsequently, working groups developed criteria for reaching domain level,

risk-of-bias judgments based on answers to signalling questions, and expanded the guidance.

The core team designed algorithms to match the criteria, which were checked by the working

groups. The resulting revision was tested in another round of piloting by 10 systematic

review authors in mid-2016.

A complete draft of version 2 of the RoB tool (RoB 2), together with detailed guidance,

was posted at www.riskofbias.info in October 2016, coinciding with the Cochrane

Colloquium in Seoul, South Korea. Feedback was invited through direct contact with the

development group. Several review teams subsequently piloted the draft tool and provided

feedback. Further modifications—particularly improvements in wording and clarity, splitting

compound signalling questions, adding new questions, and addressed methodological issues

—were made on the basis of feedback from training events (including webinars) conducted

between 2016 and 2019, as well as individual feedback from users worldwide.

Version 2 of the Cochrane tool for assessing risk of bias in randomised trials (RoB 2)

RoB 2 provides a framework for assessing the risk of bias in a single estimate of an

intervention effect reported from a randomised trial. The effect assessed is a comparison of

two interventions, which we refer to as the experimental and comparator interventions, for a

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specific outcome or endpoint. The process of making a RoB 2 assessment is summarised in

figure 1. Preliminary considerations (web appendix 1) include specifying which result is

being assessed, specifying how this result is being interpreted (see “The intervention effect of

interest” below) and listing the sources of information used to inform the assessment. Review

authors should contact trial authors in order to obtain information that is omitted from

published and online sources, so far as this is feasible. Note that risk-of-bias assessments

might be needed for results relating to multiple outcomes from the included trials.

Fig 1 Summary of the process of assessing risk of bias in a systematic review of randomised trials, using version 2 of the Cochrane risk-of-bias tool

RoB 2 is structured into five bias domains, listed in table 1. The domains were selected to

address all important mechanisms by which bias can be introduced into the results of a trial,

based on a combination of empirical evidence and theoretical considerations. We did not

include domains for features that would be expected to operate indirectly, through the

included bias domains.14 15 For this reason, we excluded some trial features, such as funding

source and single centre versus multicentre status, which have been associated empirically

with trial effect estimates from trials.

Table 1 Version 2 of the Cochrane risk-of-bias assessment tool for randomised trials: bias domains, signalling questions, response options, and risk-of-bias judgments

Bias domain and signalling question* Response optionsLower risk of

bias

Higher risk of

bias

Other

Bias arising from the randomisation process1.1 Was the allocation sequence random? Y/PY N/PN NI1.2 Was the allocation sequence concealed until participants were enrolled and assigned to interventions? Y/PY N/PN NI1.3 Did baseline differences between intervention groups suggest a problem with the randomisation process? N/PN Y/PY NIRisk-of-bias judgment (low/high/some concerns)Optional: What is the predicted direction of bias arising from the randomisation process?Bias due to deviations from intended interventions2.1 Were participants aware of their assigned intervention during the trial? N/PN Y/PY NI2.2 Were carers and people delivering the interventions aware of participants’ assigned intervention during the trial?

N/PN Y/PY NI

2.3 If Y/PY/NI to 2.1 or 2.2: Were there deviations from the intended intervention that arose because of the trial context?

N/PN Y/PY NA/NI

2.4 If Y/PY/NI to 2.3: Were these deviations likely to have affected the outcome? N/PN Y/PY NA/NI2.5 If Y/PY to 2.4: Were these deviations from intended intervention balanced between groups? Y/PY N/PN NA/NI2.6 Was an appropriate analysis used to estimate the effect of assignment to intervention? Y/PY N/PN NI2.7 If N/PN/NI to 2.6: Was there potential for a substantial impact (on the result) of the failure to analyse participants in the group to which they were randomised?

N/PN Y/PY NA/NI

Risk-of-bias judgment (low/high/some concerns)Optional: What is the predicted direction of bias due to deviations from intended interventions?

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Bias due to missing outcome data3.1 Were data for this outcome available for all, or nearly all, participants randomised? Y/PY N/PN NI3.2 If N/PN/NI to 3.1: Is there evidence that the result was not biased by missing outcome data? Y/PY N/PN NA3.3 If N/PN to 3.2: Could missingness in the outcome depend on its true value? N/PN Y/PY NA/NI3.4 If Y/PY/NI to 3.3: Is it likely that missingness in the outcome depended on its true value? N/PN Y/PY NA/NIRisk-of-bias judgment (low/high/some concerns)Optional: What is the predicted direction of bias due to missing outcome data? Bias in measurement of the outcome4.1 Was the method of measuring the outcome inappropriate? N/PN Y/PY NI4.2 Could measurement or ascertainment of the outcome have differed between intervention groups? N/PN Y/PY NI4.3 If N/PN/NI to 4.1 and 4.2: Were outcome assessors aware of the intervention received by study participants? N/PN Y/PY NI4.4 If Y/PY/NI to 4.3: Could assessment of the outcome have been influenced by knowledge of intervention received?

N/PN Y/PY NA/NI

4.5 If Y/PY/NI to 4.4: Is it likely that assessment of the outcome was influenced by knowledge of intervention received?

N/PN Y/PY NA/NI

Risk-of-bias judgment (low/high/some concerns)Optional: What is the predicted direction of bias in measurement of the outcome? Bias in selection of the reported result5.1 Were the data that produced this result analysed in accordance with a prespecified analysis plan that was finalised before unblinded outcome data were available for analysis?

Y/PY N/PN NI

Is the numerical result being assessed likely to have been selected, on the basis of the results, from: 5.2 ... multiple eligible outcome measurements (eg, scales, definitions, time points) within the outcome domain? N/PN Y/PY NI 5.3 ... multiple eligible analyses of the data? N/PN Y/PY NIRisk-of-bias judgment (low/high/some concerns)Optional: What is the predicted direction bias due to selection of the reported results?Overall biasRisk-of-bias judgment (low/high/some concerns)Optional: What is the overall predicted direction of bias for this outcome?

Y=yes; PY=probably yes; PN=probably no; N=no; NA=not applicable; NI=no information.*Signalling questions for bias due to deviations from intended interventions relate to the effect of assignment to intervention.

We label the domains using descriptions of the causes of bias addressed, avoiding terms

used in the original RoB tool (such as “selection bias” and “performance bias”) because they

are used inconsistently or not known by many people outside Cochrane.16 Each domain is

mandatory, and no others can be added, although we have developed versions of RoB 2 that

deal with additional issues that arise in trials with cluster randomised or crossover designs

(www.riskofbias.info). Within each domain, the assessment comprises:

A series of signalling questions A judgment about risk of bias for the domain, facilitated by an algorithm that maps

responses to signalling questions to a proposed judgment Free text boxes to justify responses to the signalling questions and risk-of-bias judgments Optional free text boxes to predict (and explain) the likely direction of bias.

Table 2 lists the most important changes made in RoB 2, compared with the original

Cochrane RoB tool.

Table 2 Major changes in version 2 of the Cochrane risk-of-bias assessment tool, compared with the original tool

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Bias domain Major changes compared with original risk-of-bias toolBias arising from the randomisation process

The original tool did not deal with issues relating to baseline differences. We emphasise that baseline differences that are compatible with chance do not lead to a risk of bias

Bias due to deviations from intended interventions

1. The original tool only dealt with whether participants, carers, and people delivering the interventions were aware of participants’ assigned intervention during the trial. The revised tool recognises that open trials can be at low risk of bias, if there were no deviations from intended intervention that arose because of the trial context.2. Whether the analysis was appropriate to estimate the effect of assignment to intervention was previously assessed in relation to missing outcome data.3. The original tool did not address bias in estimating the effect of adhering to intervention. Imbalances in co-interventions, failures in implementing the intervention, and non-adherences can all bias such estimates. An appropriate analysis has the potential to deal with such biases, in some circumstances.

Bias due to missing outcome data

1. Issues relating to exclusions in analyses (eg, naive per protocol analyses) are now dealt with in the “deviations from intended intervention” domain.2. Whether missing outcome data lead to bias depends on the relation between the true value of the outcome in participants with missing outcome data, and the missingness mechanism (that is, the process that led to outcome data being missing). This domain has been substantially reworked, to reflect situations in which missing outcome data do and do not lead to bias in a complete case analysis.3. We clarify that multiple imputation methods will not remove or reduce bias that occurs when missingness in the outcome depends on its true value, unless such missingness can be explained by measured variables.

Bias in measurement of the outcome

The original tool only dealt with whether outcome assessors were aware of the intervention received by study participants. This domain now covers a range of ways in which the method of outcome measurement can lead to bias, including issues related to passive detection of outcomes that might be particularly relevant for adverse effects (harms) of interventions.

Bias in selection of the reported result

1. Unlike the original tool, this domain does not deal with bias due to selective non-reporting of results (either because of non-publication of whole studies or selective reporting of outcomes) for outcome domains that were measured and analysed. Such bias puts the result of a synthesis at risk because results are omitted based on their direction, magnitude, or statistical significance. It should therefore be dealt with at the review level, as part of an integrated assessment of the risk of reporting bias.2. A judgment of low risk of bias requires that the trial was analysed in accordance with a prespecified plan that was finalised before unblinded outcome data were available for analysis.

Signalling questionsSignalling questions aim to elicit information relevant to an assessment of risk of bias

(table 1). The questions seek to be reasonably factual in nature. The response options are

“yes,” “probably yes,” “probably no,” “no,” and “no information.” To maximise their

simplicity and clarity, signalling questions are phrased such that a yes answer might indicate

either lower or higher risk of bias, depending on the most natural way to ask the question.

The online supplementary material in web appendix 2 includes elaborations providing

guidance on how to answer each question.

Responses of “yes” and “probably yes” have the same implications for risk of bias, as do

responses of “no” and “probably no.” “Yes” and “no” typically imply that firm evidence is

available; the “probably” responses typically imply that a judgment has been made. Where

there is a need to distinguish between “some concerns” and “high risk of bias,” this is dealt

with by using an additional signalling question, rather than by making a distinction between

responses “probably yes” and “yes,” or between “probably no” and “no.” The “no

information” response should be used only when insufficient details are available to allow a

different response, and when, in the absence of these details, it would be unreasonable to

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respond “probably yes” or “probably no.” For example, in the context of a large trial run by

an experienced clinical trials unit, absence of specific information about generation of the

randomisation sequence, in a paper published in a journal with rigorously enforced word

count limits, is likely to result in a response of “probably yes” rather than “no information” to

the signalling question about sequence generation (the rationale for the response should be

provided in the free text box). Some signalling questions are answered only if the response to

a previous question indicates that they are required.

The intervention effect of interestAssessments for the domain “bias due to deviations from intended interventions” differ

according to whether review authors are interested in quantifying the effect of assignment to

the interventions at baseline regardless of whether the interventions are received during

follow-up (intention-to-treat effect), or the effect of adhering to intervention as specified in

the trial protocol (per protocol effect). These effects will differ if some patients do not receive

their assigned intervention or deviate from the assigned intervention after baseline. Each

effect might be of interest.11 For example, the effect of assignment to intervention might be

appropriate to inform a health policy question about whether to recommend an intervention

(eg, a screening programme) in a particular health system, whereas the effect of adhering to

intervention more directly informs a care decision by an individual patient (eg, whether to be

screened). Changes to an intervention that are consistent with the trial protocol (even if not

explicitly discussed in the protocol), such as cessation of a drug because of toxicity or switch

to second line chemotherapy because of progression of cancer, do not cause bias and should

not be considered to be deviations from intended intervention.

The effect of assignment to intervention should be estimated by an intention-to-treat

analysis that includes all randomised participants.17 However, estimates of per protocol

effects commonly used in reports of randomised trials are problematic and might be seriously

biased.18 These estimates include those from naive per protocol analyses restricted to

individuals who adhered to their assigned intervention, and as-treated analyses in which

participants are analysed according to the intervention they received, even if their assigned

group is different. These approaches are problematic because prognostic factors could

influence whether individuals receive their allocated intervention. Data from a randomised

trial can be used to derive an unbiased estimate the effect of adhering to intervention.19 20

However, the validity of appropriate methods depends on strong assumptions, and published

applications are relatively rare to date. For trials comparing interventions that are sustained

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over time, appropriate methods also need measurement of and adjustment for the values of

prognostic factors, both before and after randomisation, that predict deviations from

intervention 11 For these reasons, most systematic reviews are likely to estimate the effect of

assignment rather than adherence to intervention.

Risk-of-bias judgmentsThe risk-of-bias judgments for each domain are “low risk of bias,” “some concerns,” or

“high risk of bias.” Judgments are based on, and summarise, the answers to signalling

questions. Review authors should interpret “risk of bias” as “risk of material bias”: concerns

should be expressed only about issues likely to have a notable effect on the result being

assessed.

An important innovation in RoB 2 is the inclusion of algorithms that map responses to

signalling questions to a proposed risk-of-bias judgment for each domain (see online

supplementary material in web appendix 2). Review authors can override these proposed

judgments if they feel it is appropriate to do so.

Free text boxes alongside the signalling questions and judgments allow assessors to

provide support for the responses. Brief direct quotations from the texts of the study reports

(including trial protocols) should be used whenever possible, supplemented by any

information obtained from authors when contacted. Reasons for any judgments that do not

follow the algorithms should be provided. RoB 2 includes optional judgments of the direction

of the bias for each domain and overall. If review authors do not have a clear rationale for

judging the likely direction of the bias, they should not guess it.

Overall risk of bias for the resultThe response options for an overall risk-of-bias judgment are the same as for individual

domains. Table 3 shows the approach to mapping bias judgments within domains to an

overall judgment for the result. The overall risk of bias generally corresponds to the worst

risk of bias in any of the domains. However, if a study is judged to have “some concerns”

about risk of bias for multiple domains, it might be judged as at high risk of bias overall.

Figure 2 shows a forest plot that displays domain specific risk of bias and overall risk of bias,

with the meta-analysis stratified by overall risk of bias.

Table 3 Approach to reaching an overall risk-of-bias judgment for a specific resultOverall risk-of-bias judgment CriteriaLow risk of bias The study is judged to be at low risk of bias for all domains for this resultSome concerns The study is judged to raise some concerns in at least one domain for this result,

but not to be at high risk of bias for any domainHigh risk of bias The study is judged to be at high risk of bias in at least one domain for this result,

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or the study is judged to have some concerns for multiple domains in a way that substantially lowers confidence in the result

Fig 2 Example forest plot showing results of a risk-of bias assessment in a systematic review of randomised trials, using version 2 of the Cochrane risk-of-bias tool. Studies are stratified by overall risk of bias

DiscussionWe have substantially revised the Cochrane tool for assessing risk of bias in the results of

randomised trials, in order to address limitations identified since it was published in 2008 and

to incorporate improvements that aim to increase the reliability of assessments. RoB 2 is

based on wide consultation within and outside Cochrane, extensive piloting, and integration

of feedback based on user experience. Assessments are made in five bias domains, within

which answers to signalling questions address a broader range of issues than in the original

RoB tool. These issues include whether post-randomisation deviations from intervention

caused bias in trials in which blinding was either not feasible or not implemented and

whether outcome data were missing for reasons likely to lead to bias. Assessment of selective

reporting is focused on a reported result for an outcome, rather than selective non-reporting

of other outcomes that were measured in the trial. RoB 2 also incorporates recent

developments in estimation of intervention effects from randomised trials: we distinguish

bias in the effect of assignment to interventions from bias in the effect of adhering to

intervention as specified in the trial protocol.11

RoB 2 assessments relate to the risk of bias in a single estimate of intervention effect for a

single outcome or endpoint, rather than for a whole trial. This specificity is because the risk

of bias is outcome specific for domains such as bias in measurement of the outcome, and

could be specific to a particular estimate (eg, when both intention-to-treat and per protocol

analyses have been conducted). We recommend that overall RoB 2 judgments of risk of bias

for individual results should be the primary means of distinguishing stronger from weaker

evidence in the context of a meta-analysis (or other synthesis) of randomised trials. The

overall judgments should also influence the strength of conclusions drawn from a systematic

review (potentially as part of a GRADE assessment).21 We strongly encourage stratification

by overall risk-of-bias judgment as a default meta-analysis strategy, as shown in figure 2. To

facilitate this, we suggest that software for systematic review preparation provides data fields

for risk-of-bias assessments. We are preparing an interactive web tool for completing RoB 2

assessments, which we hope will interface well with other systematic review software.

In RoB 2, judgments about risk of bias are derived by algorithms on the basis of answers

to specific signalling questions. The added structure provided by the signalling questions

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aims to make the assessment easier and more efficient to use, as well as to improve

agreement between assessors. We believe this approach to be more straightforward than the

direct judgments about risk of bias required in the original RoB tool. The algorithms include

explicit mappings for situations where there is no information to answer a signalling question,

which do not necessarily map to a negative assessment of the trial. For example, when

randomisation methods are described and are adequate, the response to the signalling

question about baseline imbalances between intervention groups leads to low risk of bias

either when such imbalances are compatible with chance, or when there is no information

about baseline imbalances. We removed the option of an “unclear” judgment in favour of a

graded set of response options (from “low” to “some concerns” to “high”). We envisage that

systematic reviews will report the domain level judgments and overall risk-of-bias judgments

in tables or figures contained in the main review text. In addition, we encourage reporting of

answers to signalling questions, together with direct quotes from papers and free text

justification of the answers, in an appendix.

We expect the refinements we have made to the RoB tool to lead to a greater proportion

of trial results being assessed as at low risk of bias, because our algorithms map some

circumstances to a low risk of bias when users of the previous tool would typically have

assessed them to be at unclear (or even high) risk of bias. This potential difference in

judgements in RoB 2 compared with the original tool is particularly the case for unblinded

trials, where risk of bias in the effect of assignment to intervention might be low despite

many users of the original RoB tool assigning a high risk of bias to this domain. We believe

that judgments of low risk of bias should be readily achievable for a randomised trial, a study

design that is scientifically strong, well understood, and often well implemented in practice.

We hope that RoB 2 will be useful to systematic review authors and those making use of

reviews, by providing a coherent framework for understanding and identifying trials at risk of

bias. This framework might also help those designing, conducting, and reporting randomised

trials to achieve the most reliable findings possible.

Web extra Extra material supplied by authorsWeb appendix 1: The RoB 2 tool: Preliminary considerations

file: stej049848.ww2Web appendix 2: Supplementary material

file: stej049848.ww1

We dedicate this work to Douglas G Altman, whose contributions were of fundamental importance to development of risk-of-bias assessment in systematic reviews. We thank Henning Keinke Andersen, Nancy Berkman, Mike Campbell, Rachel Churchill, Mike Clarke, Nicky Cullen, Francois Curtin, Amy Drahota, Bruno Giraudeau, Jeremy Grimshaw, Sharea

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Ijaz, Yoon Loke, Geraldine Macdonald, Richard Morris, Mona Nasser, Nishith Patel, Jani Ruotsalainen, Stephen Senn, Holger Schünemann, Nandi Siegfried, Jayne Tierney, and Sunita Vohra for contributing to discussions; and Andrew Beswick, Julia Bidonde, Angela Busch, staff at Cochrane Argentina, Karen Dawe, Franco De Crescenzo, Kristine Egberts, Clovis Mariano Faggion Jr, Clare French, Lina Gölz, Valerie Hoffman, Joni Jackson, Tim Jones, Kayleigh Kew, Elsa Marques, Silvia Minozzi, Theresa Moore, Rebecca Normansell, Rosanne Freak-Poli, Sarah Lensen, José López-López, Marlies Manders, Luke McGuinness, Spyros Papageorgiou, Melissa Randall, Phil Riley, Claudia Smeets, Meera Viswanathan, and Tanya Walsh for contributing to piloting of earlier drafts of the RoB 2 tool.

Contributors: JACS, JS, and JPTH conceived the project. JACS, JPTH, JS, MJP, and RGE oversaw the project. JACS, JS, AH, IB, BCR, and JJK led working groups. All authors contributed to development of RoB 2 and to writing associated guidance. JACS, JS, and JPTH wrote the first draft of the manuscript. All authors reviewed and commented on drafts of the manuscript. The authors are epidemiologists, statisticians, systematic reviewers, trialists, and health services researchers, many of whom are involved with Cochrane systematic reviews, methods groups, and training events. Development of RoB 2 was informed by relevant methodological literature, previously published tools for assessing methodological quality of randomised trials, systematic reviews of such tools and relevant literature, and by the authors’ experience of developing tools to assess risk of bias in randomised and non-randomised studies, diagnostic test accuracy studies, and systematic reviews. All authors contributed to development of RoB 2 and to writing associated guidance. All authors reviewed and commented on drafts of the manuscript. JACS and JPTH will act as guarantors. The corresponding author attests that all listed authors meet authorship criteria and that no others meeting the criteria have been omitted.

Funding: The development of the RoB 2 tool was supported by the UK Medical Research Council (MRC) Network of Hubs for Trials Methodology Research (MR/L004933/2- N61), with the support of the host MRC ConDuCT-II Hub (Collaboration and innovation for Difficult and Complex randomised controlled Trials In Invasive procedures, MR/K025643/1), by MRC research grant MR/M025209/1, and by a grant from the Cochrane Collaboration. JACS, SME, and JPTH are National Institute for Health Research (NIHR) senior investigators. JACS, NSB, H-YC, BCR, and JPTH are supported by NIHR Bristol Biomedical Research Centre at University Hospitals Bristol NHS Foundation Trust and the University of Bristol. JACS and JPTH are members of the MRC Integrative Epidemiology Unit at the University of Bristol. JS, PFW, and JPTH are supported by the NIHR Collaboration for Leadership in Applied Health Research and Care West (CLAHRC West) at University Hospitals Bristol NHS Foundation Trust. PJ is a tier 1 Canada Research Chair in Clinical Epidemiology of Chronic Diseases supported by the Canada Research Chairs Programme. MJP is supported by an Early Career Fellowship from the Australian National Health and Medical Research Council (NHMRC 1088535). IRW was supported by the MRC Programme MC_UU_12023/21. The views expressed in this article are those of the authors and do not necessarily represent those of the UK National Health Service, NIHR, MRC, NHMRC, or Department of Health and Social Care. Development of the RoB tool, writing the paper and the decision to submit for publication were independent of all research funders.

Competing interests: All authors have completed the ICMJE uniform disclosure form at www.icmje.org/coi_disclosure.pdf and declare: support from the MRC and Wellcome Trust for the submitted work; JACS reports grants from the MRC and NIHR during the conduct of the study; JS reports grants from the MRC, Cochrane Collaboration, and NIHR during the conduct of the study; CJC is one of the coordinating editors of Cochrane Airways and has responsibility for training Cochrane authors in the UK in Cochrane methodology (including

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http://www.icmje.org/coi_disclosure.pdf

the assessment of risks of bias); JRE reports grants from the MRC and Boehringer Ingelheim, outside the submitted work; MAH reports grants from the National Institutes of Health (NIH) during the conduct of the study; PJ serves as an unpaid member of the steering group of trials funded by Astra Zeneca, Biotronik, Biosensors, St Jude Medical, and The Medicines Company; JJK reports personal fees from BMJ outside the submitted work; TLa is an employee of Cochrane; TLi reports grants from the NIH National Eye Institute, and the Patient-Centered Outcomes Research Institute during the conduct of the study; AM reports grants from MRC and Cancer Research UK during the conduct of the study; KT reports grants from MRC during the conduct of the study and personal fees from CHDI outside the submitted work; JPTH reports grants from the MRC and Cochrane Collaboration during the conduct of the study; the authors declare no other relationships or activities that could appear to have influenced the submitted work.

Provenance and peer review: Not commissioned, externally peer reviewed.

Patient and public involvement: Patients and the public were not involved in this methodological research. We plan to disseminate the research widely, including to community participants in Cochrane.

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Revised Cochrane risk-of-bias tool for randomized trials (RoB 2): supplementary material

Domain 1: Risk of bias arising from the randomization processSignalling questions Elaboration Response

options

1.1 Was the allocation sequence random?

Answer ‘Yes’ if a random component was used in the sequence generation process. Examples include computer-generated random numbers; reference to a random number table; coin tossing; shuffling cards or envelopes; throwing dice; or drawing lots. Minimization is generally implemented with a random element (at least when the scores are equal), so an allocation sequence that is generated using minimization should generally be considered to be random.

Answer ‘No’ if no random element was used in generating the allocation sequence or the sequence is predictable. Examples include alternation; methods based on dates (of birth or admission); patient record numbers; allocation decisions made by clinicians or participants; allocation based on the availability of the intervention; or any other systematic or haphazard method.Answer ‘No information’ if the only information about randomization methods is a statement that the study is randomized.

In some situations a judgement may be made to answer ‘Probably no’ or ‘Probably yes’. For example, , in the context of a large trial run by an experienced clinical trials unit, absence of specific information about generation of the randomization sequence, in a paper published in a journal with rigorously enforced word count limits, is likely to result in a response of ‘Probably yes’ rather than ‘No information’. Alternatively, if other (contemporary) trials by the same investigator team have clearly used non-random sequences, it might be reasonable to assume that the current study was done using similar methods.

Y/PY/PN/N/NI

1.2 Was the allocation sequence concealed until participants were enrolled and assigned to interventions?

Answer ‘Yes’ if the trial used any form of remote or centrally administered method to allocate interventions to participants, where the process of allocation is controlled by an external unit or organization, independent of the enrolment personnel (e.g. independent central pharmacy, telephone or internet-based randomization service providers).

Answer ‘Yes’ if envelopes or drug containers were used appropriately. Envelopes should be opaque, sequentially numbered, sealed with a tamper-proof seal and opened only after the envelope has been irreversibly assigned to the participant. Drug containers should be sequentially numbered and of identical appearance, and dispensed or administered only after they have been irreversibly assigned to the participant. This level of detail is rarely provided in reports, and a judgement may be required to justify an answer of ‘Probably yes’ or ‘Probably no’.Answer ‘No’ if there is reason to suspect that the enrolling investigator or the participant had knowledge of the forthcoming allocation.

Y/PY/PN/N/NI

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1.3 Did baseline differences between intervention groups suggest a problem with the randomization process?

Note that differences that are compatible with chance do not lead to a risk of bias. A small number of differences identified as ‘statistically significant’ at the conventional 0.05 threshold should usually be considered to be compatible with chance.

Answer ‘No’ if no imbalances are apparent or if any observed imbalances are compatible with chance.Answer ‘Yes’ if there are imbalances that indicate problems with the randomization process, including:

(1) substantial differences between intervention group sizes, compared with the intended allocation ratio;1. or(2) a substantial excess in statistically significant differences in baseline characteristics between intervention groups, beyond

that expected by chance; or(3) imbalance in one or more key prognostic factors, or baseline measures of outcome variables, that is very unlikely to be

due to chance and for which the between-group difference is big enough to result in bias in the intervention effect estimate.

Also answer ‘Yes’ if there are other reasons to suspect that the randomization process was problematic:

(4) excessive similarity in baseline characteristics that is not compatible with chance.

Answer ‘No information’ when there is no useful baseline information available (e.g. abstracts, or studies that reported only baseline characteristics of participants in the final analysis). The answer to this question should not influence answers to questions 1.1 or 1.2. For example, if the trial has large baseline imbalances, but authors report adequate randomization methods, questions 1.1 and 1.2 should still be answered on the basis of the reported adequate methods, and any concerns about the imbalance should be raised in the answer to the question 1.3 and reflected in the domain-level risk-of-bias judgement.Trialists may undertake analyses that attempt to deal with flawed randomization by controlling for imbalances in prognostic factors at baseline. To remove the risk of bias caused by problems in the randomization process, it would be necessary to know, and measure, all the prognostic factors that were imbalanced at baseline. It is unlikely that all important prognostic factors are known and measured, so such analyses will at best reduce the risk of bias. If review authors wish to assess the risk of bias in a trial that controlled for baseline imbalances in order to mitigate failures of randomization, the study should be assessed using the ROBINS-I tool.

Y/PY/PN/N/NI

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Risk-of-bias judgement

See table below. Low / High / Some concerns

Optional: What is the predicted direction of bias arising from the randomization process?

If the likely direction of bias can be predicted, it is helpful to state this. The direction might be characterized either as being towards (or away from) the null, or as being in favour of one of the interventions.

NA / Favours experimental /

Favours comparator / Towards null

/Away from null / Unpredictable

Algorithm for suggested judgment of risk of bias arising from the randomization process

Low risk of bias (i) The allocation sequence was adequately concealedAND

(ii.1) Any baseline differences observed between intervention groups appear to be compatible with chanceOR(ii.2) There is no information about baseline imbalances

AND(iii.1) The allocation sequence was randomOR

(iii.2) There is no information about whether the allocation sequence was randomSome concerns (i.1) The allocation sequence was adequately concealed

AND(i.2.1) The allocation sequence was not randomOR(i.2.2) Baseline differences between intervention groups suggest a problem with the randomization process

OR(ii.1) There is no information about concealment of the allocation sequenceAND(ii.2) Any baseline differences observed between intervention groups appear to be compatible with chance

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OR(iii) There is no information to answer any of the signalling questions

High risk of bias (i) The allocation sequence was not adequately concealedOR

(ii.1) There is no information about concealment of the allocation sequenceAND

(ii.2) Baseline differences between intervention groups suggest a problem with the randomization process

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Domain 2: Risk of bias due to deviations from the intended interventions (effect of assignment to intervention)Signalling questions Elaboration Response options

2.1. Were participants aware of their assigned intervention during the trial?

If participants are aware of their assigned intervention it is more likely that health-related behaviours will differ between the intervention groups. Blinding participants, most commonly through use of a placebo or sham intervention, may prevent such differences. If participants experienced side effects or toxicities that they knew to be specific to one of the interventions, answer this question ‘Yes’ or ‘Probably yes’.

Y/PY/PN/N/NI

2.2. Were carers and people delivering the interventions aware of participants' assigned intervention during the trial?

If carers or people delivering the interventions are aware of the assigned intervention then its implementation, or administration of non-protocol interventions, may differ between the intervention groups. Blinding may prevent such differences. If participants experienced side effects or toxicities that carers or people delivering the interventions knew to be specific to one of the interventions, answer question ‘Yes’ or ‘Probably yes’. If randomized allocation was not concealed, then it is likely that carers and people delivering the interventions were aware of participants' assigned intervention during the trial.

Y/PY/PN/N/NI

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2.3. If Y/PY /NI to 2.1 or 2.2: Were there deviations from the intended intervention that arose because of the trial context?

For the effect of assignment to intervention, this domain assesses problems that arise when changes from assigned intervention that are inconsistent with the trial protocol arose because of the trial context. We use the term trial context to refer to effects of recruitment and engagement activities on trial participants and when trial personnel (carers or people delivering the interventions) undermine the implementation of the trial protocol in ways that would not happen outside the trial. For example, the process of securing informed consent may lead participants subsequently assigned to the comparator group to feel unlucky and therefore seek the experimental intervention, or other interventions that improve their prognosis.Answer ‘Yes’ or ‘Probably yes’ only if there is evidence, or strong reason to believe, that the trial context led to failure to implement the protocol interventions or to implementation of interventions not allowed by the protocol.Answer ‘No’ or ‘Probably no’ if there were changes from assigned intervention that are inconsistent with the trial protocol, such as non-adherence to intervention, but these are consistent with what could occur outside the trial context.

Answer ‘No’ or ‘Probably no’ for changes to intervention that are consistent with the trial protocol, for example cessation of a drug intervention because of acute toxicity or use of additional interventions whose aim is to treat consequences of one of the intended interventions

If blinding is compromised because participants report side effects or toxicities that are specific to one of the interventions, answer ‘Yes’ or ‘Probably yes’ only if there were changes from assigned intervention that are inconsistent with the trial protocol and arose because of the trial context.

The answer ‘No information’ may be appropriate, because trialists do not always report whether deviations arose because of the trial context.

NA/Y/PY/PN/N/NI

2.4 If Y/PY /NI to 2.3 : Were these deviations likely to have affected the outcome?

Changes from assigned intervention that are inconsistent with the trial protocol and arose because of the trial context will be important if they affect the outcome, but not otherwise.

NA/Y/PY/PN/N/NI

2.5. If Y/PY to 2.4 : Were these deviations from intended intervention balanced between groups?

Changes from assigned intervention that are inconsistent with the trial protocol and arose because of the trial context are more likely to lead to bias if they are not balanced between the intervention groups.

NA/Y/PY/PN/N/NI

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2.6 Was an appropriate analysis used to estimate the effect of assignment to intervention?

Both intention-to-treat (ITT) analyses and modified intention-to-treat (mITT) analyses excluding participants with missing outcome data should be considered appropriate. Both naïve ‘per-protocol’ analyses (excluding trial participants who did not receive their assigned intervention) and ‘as treated’ analyses (in which trial participants are grouped according to the intervention that they received, rather than according to their assigned intervention) should be considered inappropriate. Analyses excluding eligible trial participants post-randomization should also be considered inappropriate, but post-randomization exclusions of ineligible participants (when eligibility was not confirmed until after randomization, and could not have been influenced by intervention group assignment) can be considered appropriate.

Y/PY/PN/N/NI

2.7 If N/PN /NI to 2.6: Was there potential for a substantial impact (on the result) of the failure to analyse participants in the group to which they were randomized?

This question addresses whether the number of participants who were analysed in the wrong intervention group, or excluded from the analysis, was sufficient that there could have been a substantial impact on the result. It is not possible to specify a precise rule: there may be potential for substantial impact even if fewer than 5% of participants were analysed in the wrong group or excluded, if the outcome is rare or if exclusions are strongly related to prognostic factors.

NA/Y/PY/PN/N/NI

Risk-of-bias judgement See table below. Low / High / Some concerns

Optional: What is the predicted direction of bias due to deviations from intended interventions?


NA / Favours experimental / Favours comparator / Towards null /Away from null /

Unpredictable

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Algorithm for suggested judgment of risk of bias due to deviations from the intended interventions (effect of assignment to intervention)

Because the domain addresses two somewhat distinct issues, we separate the algorithm into two parts and combine them to reach the judgement.

Part 1: criteria for questions 2.1 to 2.5 Part 2: criteria for questions 2.6 and 2.7 Criteria for the domain

Low risk of bias (i) Participants, carers and people delivering the interventions were unaware of intervention groups during the trialOR

(ii.1) Participants, carers or people delivering the interventions were aware of intervention groups during the trial AND(ii.2) No deviations from intended intervention arose because of the trial context.

An appropriate analysis was used to estimate the effect of assignment to intervention

‘Low’ risk of bias for Part 1 AND‘Low’ risk of bias for Part 2

Some concerns (i) Participants, carers or people delivering the interventions were aware of intervention groups during the trialAND

(ii.1) There is no information on whether there were deviations from intended intervention because of the trial contextOR

(ii.1.1) There were deviations from intended interventions that arose because of the trial contextAND

(ii.1.1.1) These deviations were not likely to have affected the outcome

(i) An appropriate analysis was not used to estimate the effect of assignment to interventionAND(ii) The potential impact (on the estimated effect of intervention) of the failure to analyse participants in the group to which they were randomized was not substantial

‘‘Some concerns’ for Part 1OR‘‘Some concerns’ for Part 2

ANDPart 1 not ‘High’ risk of biasANDPart 2 not ‘High’ risk of bias

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OR(ii.1.1.2) These deviations were balanced between the intervention groups

High risk of bias (i) Participants, carers or people delivering the interventions were aware of intervention groups during the trialAND(ii) There were deviations from intended interventions that arose because of the contextAND(iii) These deviations were likely to have affected the outcomeAND(iv) These deviations were unbalanced between the intervention groups

(i) An appropriate analysis was not used to estimate the effect of assignment to interventionAND(ii) The potential impact (on the estimated effect of intervention) of the failure to analyse participants in the group to which they were randomized was substantial

‘High’ risk of bias for Part 1 OR‘High’ risk of bias for Part 2

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Domain 2: Risk of bias due to deviations from the intended interventions (effect of adhering to intervention)Signalling questions Elaboration Response options

2.1. Were participants aware of their assigned intervention during the trial?

If participants are aware of their assigned intervention it is more likely that health-related behaviours will differ between the intervention groups. Blinding participants, most commonly through use of a placebo or sham intervention, may prevent such differences. If participants experienced side effects or toxicities that they knew to be specific to one of the interventions, answer this question ‘Yes’ or ‘Probably yes’.

Y/PY/PN/N/NI

2.2. Were carers and people delivering the interventions aware of participants' assigned intervention during the trial?

If carers or people delivering the interventions are aware of the assigned intervention then its implementation, or administration of non-protocol interventions, may differ between the intervention groups. Blinding may prevent such differences. If participants experienced side effects or toxicities that carers or people delivering the interventions knew to be specific to one of the interventions, answer ‘Yes’ or ‘Probably yes’. If randomized allocation was not concealed, then it is likely that carers and people delivering the interventions were aware of participants' assigned intervention during the trial.

Y/PY/PN/N/NI

2.3. [If applicable:] If Y/PY /NI to 2.1 or 2.2 : Were important non-protocol interventions balanced across intervention groups?

This question is asked only if the preliminary considerations specify that the assessment will address imbalance of important non-protocol interventions between intervention groups. Important non-protocol interventions are the additional interventions or exposures that: (1) are inconsistent with the trial protocol; (2) trial participants might receive with or after starting their assigned intervention; and (3) are prognostic for the outcome. Risk of bias will be higher if there is imbalance in such interventions between the intervention groups.

NA/ Y/PY /PN/N/NI

2.4. [If applicable:] Were there failures in implementing the intervention that could have affected the outcome?

This question is asked only if the preliminary considerations specify that the assessment will address failures in implementing the intervention that could have affected the outcome. Risk of bias will be higher if the intervention was not implemented as intended by, for example, the health care professionals delivering care. Answer ‘No’ or ‘Probably no’ if implementation of the intervention was successful for most participants.

NA/Y/PY/PN/N/NI

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2.5. [If applicable:] Was there non-adherence to the assigned intervention regimen that could have affected participants’ outcomes?

This question is asked only if the preliminary considerations specify that the assessment will address non-adherence that could have affected participants’ outcomes. Non-adherence includes imperfect compliance with a sustained intervention, cessation of intervention, crossovers to the comparator intervention and switches to another active intervention. Consider available information on the proportion of study participants who continued with their assigned intervention throughout follow up, and answer ‘Yes’ or ‘Probably yes’ if the proportion who did not adhere is high enough to raise concerns. Answer ‘No’ for studies of interventions that are administered once, so that imperfect adherence is not possible, and all or most participants received the assigned intervention.

NA/Y/PY/PN/N/NI

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2.6. If N/PN /NI to 2.3, or Y/PY /NI to 2.4 or 2.5 : Was an appropriate analysis used to estimate the effect of adhering to the intervention?

Both ‘ naïve ‘per-protocol’ analyses (excluding trial participants who did not receive their allocated intervention) and ‘as treated’ analyses (comparing trial participants according to the intervention they actually received) will usually be inappropriate for estimating the effect of adhering to intervention (the ‘per-protocol’ effect). However, it is possible to use data from a randomized trial to derive an unbiased estimate of the effect of adhering to intervention. Examples of appropriate methods include: (1) instrumental variable analyses to estimate the effect of receiving the assigned intervention in trials in which a single intervention, administered only at baseline and with all-or-nothing adherence, is compared with standard care; and (2) inverse probability weighting to adjust for censoring of participants who cease adherence to their assigned intervention, in trials of sustained treatment strategies. These methods depend on strong assumptions, which should be appropriate and justified if the answer to this question is ‘Yes’ or ‘Probably yes’. It is possible that a paper reports an analysis based on such methods without reporting information on the deviations from intended intervention, but it would be hard to judge such an analysis to be appropriate in the absence of such information.

If an important non-protocol intervention was administered to all participants in one intervention group, adjustments cannot be made to overcome this.

Some examples of analysis strategies that would not be appropriate to estimate the effect of adhering to intervention are (i) ‘Intention to treat (ITT) analysis’, (ii) ‘per protocol analysis’, (iii) ‘as-treated analysis’, (iv) ‘analysis by treatment received’.

NA/Y/PY/PN/N/NI

Risk-of-bias judgement See algorithm. Low / High / Some concerns

Optional: What is the predicted direction of bias due to deviations from intended interventions?



Unpredictable

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Algorithm for suggested judgment of risk of bias due to deviations from the intended interventions (effect of adhering to intervention)

Low risk of bias (i.1) Participants, carers and people delivering the interventions were unaware of intervention groups during the trial

OR(i.2.1) Participants, carers or people delivering the interventions were aware of intervention groups

AND(i.2.2) [If applicable] The important non-protocol interventions were balanced across intervention groups

AND(ii) [If applicable] Failures in implementing the intervention could not have affected the outcome

AND(iii) [If applicable] Study participants adhered to the assigned intervention regimen

Some concerns (i.1.1) Participants, carers and people delivering the interventions were unaware of intervention groups during the trial AND

(i.1.2.1) [If applicable] Failures in implementing the intervention could have affected the outcome OR

(i.1.2.2) [If applicable] Study participants did not adhere to the assigned intervention regimenOR

(i.2.1) Participants, carers or people delivering the interventions were aware of intervention groups AND

(i.2.2) [If applicable] The important non-protocol interventions were balanced across intervention groupsAND




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(i.3.2) [If applicable] The important non-protocol interventions were not balanced across intervention groupsAND

(ii) An appropriate analysis was used to estimate the effect of adhering to the intervention

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High risk of bias (i.1.1) Participants, carers and people delivering the interventions were unaware of intervention groups during the trial AND




(i.2.2) [If applicable] The important non-protocol interventions were balanced across intervention groupsAND




(i.3.2) [If applicable] The important non-protocol interventions were not balanced across intervention groupsAND

(ii) An appropriate analysis was not used to estimate the effect of adhering to the intervention

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Domain 3: Risk of bias due to missing outcome dataSignalling questions Elaboration Response options

3.1 Were data for this outcome available for all, or nearly all, participants randomized?

The appropriate study population for an analysis of the intention to treat effect is all randomized participants.

“Nearly all” should be interpreted as that the number of participants with missing outcome data is sufficiently small that their outcomes, whatever they were, could have made no important difference to the estimated effect of intervention.For continuous outcomes, availability of data from 95% of the participants will often be sufficient. For dichotomous outcomes, the proportion required is directly linked to the risk of the event. If the observed number of events is much greater than the number of participants with missing outcome data, the bias would necessarily be small. Only answer ‘No information’ if the trial report provides no information about the extent of missing outcome data. This situation will usually lead to a judgement that there is a high risk of bias due to missing outcome data.Note that imputed data should be regarded as missing data, and not considered as ‘outcome data’ in the context of this question.

Y/PY/PN/N/NI

3.2 If N/PN /NI to 3.1 : Is there evidence that the result was not biased by missing outcome data?

Evidence that the result was not biased by missing outcome data may come from: (1) analysis methods that correct for bias; or (2) sensitivity analyses showing that results are little changed under a range of plausible assumptions about the relationship between missingness in the outcome and its true value. However, imputing the outcome variable, either through methods such as ‘last-observation-carried-forward’ or via multiple imputation based only on intervention group, should not be assumed to correct for bias due to missing outcome data.

NA/Y/PY/PN/N

3.3 If N/PN to 3.2 : Could missingness in the outcome depend on its true value?

If loss to follow up, or withdrawal from the study, could be related to participants’ health status, then it is possible that missingness in the outcome was influenced by its true value. However, if all missing outcome data occurred for documented reasons that are unrelated to the outcome then the risk of bias due to missing outcome data will be low (for example, failure of a measuring device or interruptions to routine data collection).

In time-to-event analyses, participants censored during trial follow-up, for example because they withdrew from the study, should be regarded as having missing outcome data, even though some of their follow up is included in the analysis. Note that such participants may be shown as included in analyses in CONSORT flow diagrams.

NA/Y/PY/PN/N/NI

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3.4 If Y/PY /NI to 3.3 : Is it likely that missingness in the outcome depended on its true value?

This question distinguishes between situations in which (i) missingness in the outcome could depend on its true value (assessed as ‘Some concerns’) from those in which (ii) it is likely that missingness in the outcome depended on its true value (assessed as ‘High risk of bias’). Five reasons for answering ‘Yes’ are:1. Differences between intervention groups in the proportions of missing outcome data. If there is a difference

between the effects of the experimental and comparator interventions on the outcome, and the missingness in the outcome is influenced by its true value, then the proportions of missing outcome data are likely to differ between intervention groups. Such a difference suggests a risk of bias due to missing outcome data, because the trial result will be sensitive to missingness in the outcome being related to its true value. For time-to-event-data, the analogue is that rates of censoring (loss to follow-up) differ between the intervention groups.

2. Reported reasons for missing outcome data provide evidence that missingness in the outcome depends on its true value;

3. Reported reasons for missing outcome data differ between the intervention groups;

4. The circumstances of the trial make it likely that missingness in the outcome depends on its true value. For example, in trials of interventions to treat schizophrenia it is widely understood that continuing symptoms make drop out more likely.

5. In time-to-event analyses, participants’ follow up is censored when they stop or change their assigned intervention, for example because of drug toxicity or, in cancer trials, when participants switch to second-line chemotherapy.6. Answer ‘No’ if the analysis accounted for participant characteristics that are likely to explain the

relationship between missingness in the outcome and its true value.

NA/Y/PY/PN/N/NI


Optional: What is the predicted direction of bias due to missing outcome data?



Unpredictable

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Suggested judgment of risk of bias due to missing outcome data

Low risk of bias (i) Outcome data were available for all, or nearly all, randomized participants OR(ii) There is evidence that the result was not biased by missing outcome data OR(iii) Missingness in the outcome could not depend on its true value

Some concerns (i) Outcome data were not available for all, or nearly all, randomized participants AND(ii) There is not evidence that the result was not biased by missing outcome data AND(iii) Missingness in the outcome could depend on its true value AND(iv) It is not likely that missingness in the outcome depended on its true value

High risk of bias (i) Outcome data were not available for all, or nearly all, randomized participants AND(ii) There is not evidence that the result was not biased by missing outcome data AND(iii) Missingness in the outcome could depend on its true value AND(iv) It is likely that missingness in the outcome depended on its true value.

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Domain 4: Risk of bias in measurement of the outcomeSignalling questions Elaboration Response options

4.1 Was the method of measuring the outcome inappropriate?

This question aims to identify methods of outcome measurement (data collection) that are unsuitable for the outcome they are intended to evaluate. The question does not aim to assess whether the choice of outcome being evaluated was sensible (e.g. because it is a surrogate or proxy for the main outcome of interest). In most circumstances, for pre-specified outcomes, the answer to this question will be ‘No’ or ‘Probably no’. Answer ‘Yes’ or ‘Probably yes’ if the method of measuring the outcome is inappropriate, for example because:

(1) it is unlikely to be sensitive to plausible intervention effects (e.g. important ranges of outcome values fall outside levels that are detectable using the measurement method); or

(2) the measurement instrument has been demonstrated to have poor validity.

Y/PY/PN/N/NI

4.2 Could measurement or ascertainment of the outcome have differed between intervention groups?

Comparable methods of outcome measurement (data collection) involve the same measurement methods and thresholds, used at comparable time points. Differences between intervention groups may arise because of ‘diagnostic detection bias’ in the context of passive collection of outcome data, or if an intervention involves additional visits to a healthcare provider, leading to additional opportunities for outcome events to be identified.

Y/PY/PN/N/NI

4.3 If N/PN /NI to 4.1 and 4.2: Were outcome assessors aware of the intervention received by study participants?

Answer ‘No’ if outcome assessors were blinded to intervention status. For participant-reported outcomes, the outcome assessor is the study participant.

Y/PY/PN/N/NI

4.4 If Y/PY /NI to 4.3 : Could assessment of the outcome have been influenced by knowledge of intervention received?

Knowledge of the assigned intervention could influence participant-reported outcomes (such as level of pain), observer-reported outcomes involving some judgement, and intervention provider decision outcomes. They are unlikely to influence observer-reported outcomes that do not involve judgement, for example all-cause mortality.

NA/Y/PY/PN/N/NI

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4.5 If Y/PY /NI to 4.4 : Is it likely that assessment of the outcome was influenced by knowledge of intervention received?

This question distinguishes between situations in which (i) knowledge of intervention status could have influenced outcome assessment but there is no reason to believe that it did (assessed as ‘Some concerns’) from those in which (ii) knowledge of intervention status was likely to influence outcome assessment (assessed as ‘High’). When there are strong levels of belief in either beneficial or harmful effects of the intervention, it is more likely that the outcome was influenced by knowledge of the intervention received. Examples may include patient-reported symptoms in trials of homeopathy, or assessments of recovery of function by a physiotherapist who delivered the intervention.

NA/Y/PY/PN/N/NI


Optional: What is the predicted direction of bias in measurement of the outcome?



Unpredictable

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Suggested judgment of risk of bias in measurement of the outcome

Low risk of bias (i) The method of measuring the outcome was not inappropriate

AND(ii) The measurement or ascertainment of the outcome did not differ between intervention groups

AND(iii.1) The outcome assessors were unaware of the intervention received by study participants

OR(iii.2) The assessment of the outcome could not have been influenced by knowledge of the intervention received

Some concerns (i.1) The method of measuring the outcome was not inappropriateAND

(i.2) The measurement or ascertainment of the outcome did not differ between intervention groupsAND

(i.3) The assessment of the outcome could have been influenced by knowledge of the intervention receivedAND

(i.4) It is unlikely that assessment of the outcome was influenced by knowledge of intervention receivedOR

(ii.1) The method of measuring the outcome was not inappropriateAND

(ii.2) There is no information on whether the measurement or ascertainment of the outcome could have differed between intervention groups

AND(ii.3.1) The outcome assessors were unaware of the intervention received by study participants

OR(ii.3.2) The assessment of the outcome could not have been influenced by knowledge of the intervention received

High risk of bias (i) The method of measuring the outcome was inappropriateOR

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(ii) The measurement or ascertainment of the outcome could have differed between intervention groupsOR

(iii) It is likely that assessment of the outcome was influenced by knowledge of the intervention received

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Domain 5: Risk of bias in selection of the reported resultSignalling questions Elaboration Response options5.1 Were the data that produced this result analysed in accordance with a pre-specified analysis plan that was finalized before unblinded outcome data were available for analysis?

If the researchers’ pre-specified intentions are available in sufficient detail, then planned outcome measurements and analyses can be compared with those presented in the published report(s). To avoid the possibility of selection of the reported result, finalization of the analysis intentions must precede availability of unblinded outcome data to the trial authors.Changes to analysis plans that were made before unblinded outcome data were available, or that were clearly unrelated to the results (e.g. due to a broken machine making data collection impossible) do not raise concerns about bias in selection of the reported result.

Y/PY/PN/N/NI

Is the numerical result being assessed likely to have been selected, on the basis of the results, from...

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5.2. ... multiple eligible outcome measurements (e.g. scales, definitions, time points) within the outcome domain?

A particular outcome domain (i.e. a true state or endpoint of interest) may be measured in multiple ways. For example, the domain pain may be measured using multiple scales (e.g. a visual analogue scale and the McGill Pain Questionnaire), each at multiple time points (e.g. 3, 6 and 12 weeks post-treatment). If multiple measurements were made, but only one or a subset is reported on the basis of the results (e.g. statistical significance), there is a high risk of bias in the fully reported result. Attention should be restricted to outcome measurements that are eligible for consideration by the RoB 2 tool user. For example, if only a result using a specific measurement scale is eligible for inclusion in a meta-analysis (e.g. Hamilton Depression Rating Scale), and this is reported by the trial, then there would not be an issue of selection even if this result was reported (on the basis of the results) in preference to the result from a different measurement scale (e.g. Beck Depression Inventory).

Answer ‘Yes’ or ‘Probably yes’ if:There is clear evidence (usually through examination of a trial protocol or statistical analysis plan) that a domain was measured in multiple eligible ways, but data for only one or a subset of measures is fully reported (without justification), and the fully reported result is likely to have been selected on the basis of the results. Selection on the basis of the results can arise from a desire for findings to be newsworthy, sufficiently noteworthy to merit publication, or to confirm a prior hypothesis. For example, trialists who have a preconception, or vested interest in showing, that an experimental intervention is beneficial may be inclined to report outcome measurements selectively that are favourable to the experimental intervention.

Answer ‘No’ or ‘Probably no’ if:There is clear evidence (usually through examination of a trial protocol or statistical analysis plan) that all eligible reported results for the outcome domain correspond to all intended outcome measurements.or

There is only one possible way in which the outcome domain can be measured (hence there is no opportunity to select from multiple measures).

orOutcome measurements are inconsistent across different reports on the same trial, but the trialists have provided the reason for the inconsistency and it is not related to the nature of the results.

Answer ‘No information’ if:

Analysis intentions are not available, or the analysis intentions are not reported in sufficient detail to enable an assessment, and there is more than one way in which the outcome domain could have been measured.

Y/PY/PN/N/NI

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5.3 ... multiple eligible analyses of the data?

A particular outcome measurement may be analysed in multiple ways. Examples include: unadjusted and adjusted models; final value vs change from baseline vs analysis of covariance; transformations of variables; different definitions of composite outcomes (e.g. ‘major adverse event’); conversion of continuously scaled outcome to categorical data with different cut-points; different sets of covariates for adjustment; and different strategies for dealing with missing data. Application of multiple methods generates multiple effect estimates for a specific outcome measurement. If multiple estimates are generated but only one or a subset is reported on the basis of the results (e.g. statistical significance), there is a high risk of bias in the fully reported result. Attention should be restricted to analyses that are eligible for consideration by the RoB 2 tool user. For example, if only the result from an analysis of post-intervention values is eligible for inclusion in a meta-analysis (e.g. at 12 weeks after randomization), and this is reported by the trial, then there would not be an issue of selection even if this result was reported (on the basis of the results) in preference to the result from an analysis of changes from baseline.Answer ‘Yes’ or ‘Probably yes’ if:

There is clear evidence (usually through examination of a trial protocol or statistical analysis plan) that a measurement was analysed in multiple eligible ways, but data for only one or a subset of analyses is fully reported (without justification), and the fully reported result is likely to have been selected on the basis of the results. Selection on the basis of the results arises from a desire for findings to be newsworthy, sufficiently noteworthy to merit publication, or to confirm a prior hypothesis. For example, trialists who have a preconception or vested interest in showing that an experimental intervention is beneficial may be inclined to selectively report analyses that are favourable to the experimental intervention.

Answer ‘No’ or ‘Probably no’ if:

There is clear evidence (usually through examination of a trial protocol or statistical analysis plan) that all eligible reported results for the outcome measurement correspond to all intended analyses.

orThere is only one possible way in which the outcome measurement can be analysed (hence there is no opportunity to select from multiple analyses).or

Analyses are inconsistent across different reports on the same trial, but the trialists have provided the reason for the inconsistency and it is not related to the nature of the results.

Answer ‘No information’ if:Analysis intentions are not available, or the analysis intentions are not reported in sufficient detail to enable an assessment, and there is more than one way in which the outcome measurement could have been analysed.

Y/PY/PN/N/NI


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Optional: What is the predicted direction of bias due to selection of the reported result?



Unpredictable

Suggested judgment of risk of bias in selection of the reported result

Low risk of bias (i) The data were analysed in accordance with a pre-specified plan that was finalized before unblinded outcome data were available for analysisAND(ii) The result being assessed is unlikely to have been selected, on the basis of the results, from multiple outcome measurements (e.g. scales, definitions, time points) within the outcome domainAND(iii) Reported outcome data are unlikely to have been selected, on the basis of the results, from multiple analyses of the data

Some concerns (i.1) The data were not analysed in accordance with a pre-specified plan that was finalized before unblinded outcome data were available for analysisAND(i.2) The result being assessed is unlikely to have been selected, on the basis of the results, from multiple outcome measurements (e.g. scales, definitions, time points) within the outcome domainAND(i.3) The result being assessed is unlikely to have been selected, on the basis of the results, from multiple analyses of the data

OR(ii)There is no information on whether the result being assessed is likely to have been selected, on the basis of the results, from multiple outcome measurements (e.g. scales, definitions, time points) within the outcome domain AND from multiple analyses of the data

High risk of bias (i) The result being assessed is likely to have been selected, on the basis of the results, from multiple outcome measurements (e.g. scales, definitions, time points) within the outcome domain OR (ii) The result being assessed is likely to have been selected, on the basis of the results, from multiple analyses of the data

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