Home > Part 2: General methods for Cochrane reviews > 8 Assessing risk of bias in included studies
Editors: Julian PT Higgins, Douglas G Altman and Jonathan AC Sterne on behalf of the Cochrane Statistical Methods Group and the Cochrane Bias Methods Group.
Key points
Problems with the design and execution of individual studies of healthcare interventions raise questions about the validity of their findings; empirical evidence provides support for this concern.
An assessment of the validity of studies included in a Cochrane review should emphasize the risk of bias in their results, i.e. the risk that they will overestimate or underestimate the true intervention effect.
Numerous tools are available for assessing methodological quality of clinical trials. We recommend against the use of scales yielding a summary score.
The Cochrane Collaboration recommends a specific tool for assessing risk of bias in each included study. This comprises a judgement and a support for the judgement for each entry in a ‘Risk of bias’ table, where each entry addresses a specific feature of the study. The judgement for each entry involves assessing the risk of bias as ‘low risk’, as ‘high risk, or as ‘unclear risk’, with the last category indicating either lack of information or uncertainty over the potential for bias.
Plots of ‘Risk of bias’ assessments can be created in RevMan.
In clinical trials, biases can be broadly categorized as selection bias, performance bias, detection bias, attrition bias, reporting bias and other biases that do not fit into these categories.
For parallel group trials, the features of interest in a standard ‘Risk of bias’ table of a Cochrane review are sequence generation (selection bias), allocation sequence concealment (selection bias), blinding of participants and personnel (performance bias), blinding of outcome assessment (detection bias), incomplete outcome data (attrition bias), selective outcome reporting (reporting bias) and other potential sources of bias.
Detailed considerations for the assessment of these features are provided in this chapter.
8.3 Tools for assessing quality and risk of bias
8.4 Introduction to sources of bias in clinical trials
Table 8.4.a: A common classification scheme for bias
8.5 The Cochrane Collaboration’s tool for assessing risk of bias
8.6 Presentation of assessments of risk of bias
Figure 8.6.a: Example of a ‘Risk of bias’ table
Figure 8.6.b: Example of a ‘Risk of bias graph’ Figure
Figure 8.6.c: Example of a ‘Risk of bias summary’ Figure
8.7 Summary assessments of risk of bias
Table 8.7.a: Possible approach for summary assessments
8.8 Incorporating assessments into analyses
8.10 Allocation sequence concealment
8.11 Blinding of participants and personnel
8.12 Blinding of outcome assessment
8.14 Selective outcome reporting
8.15 Other potential threats to validity