Tool to Assess Risk of Bias in Cohort Studies

Tool to Assess Risk of Bias in Cohort Studies

This tool has been contributed by the CLARITY Group at McMaster University
It can be downloaded from Evidence Partners: Link to tools

1. Was selection of exposed and non-exposed cohorts drawn from the same population?

 Definitely yes                    Probably yes            Probably no              Definitely no
            (low risk of bias)                                                                                 (high risk of bias)

Examples of low risk of bias:  Exposed and unexposed drawn for same administrative data base of patients presenting at same points of care over the same time frame

Examples of high risk of bias: exposed and unexposed presenting to different points of care or over a different time frame

2. Can we be confident in the assessment of exposure?

Definitely yes                     Probably yes            Probably no              Definitely no
           (low risk of bias)                                                                                 (high risk of bias)

Examples of low risk of bias: Secure record [e.g. surgical records, pharmacy records]; Repeated interview or other ascertainment asking about current use/exposure

Examples of higher risk of bias: Structured interview at a single point in time; Written self report; Individuals who are asked to retrospectively confirm their exposure status may be subject to recall bias – less likely to recall an exposure if they have not developed an adverse outcome, and more likely to recall an exposure (whether an exposure occurred or not) if they have developed an adverse outcome.

Examples of high risk of bias: uncertain how exposure information obtained

3. Can we be confident that the outcome of interest was not present at start of study 

Definitely yes                     Probably yes            Probably no              Definitely no
           (low risk of bias)                                                                                        (high risk of bias)

4. Did the study match exposed and unexposed for all variables that are associated with the outcome of interest or did the statistical analysis adjust for these prognostic variables?

Definitely yes                     Mostly yes                Mostly no                 Definitely no
           (low risk of bias)                                                                                      (high risk of bias)

Examples of low risk of bias: comprehensive matching or adjustment for all plausible prognostic variables

Examples of higher risk of bias: matching or adjustment for most plausible prognostic variables

Examples of high risk of bias: matching or adjustment for a minority of plausible prognostic variables, or no matching or adjustment at all.  Statements of no differences between groups or that differences were not statistically significant are not sufficient for establishing comparability.  

5. Can we be confident in the assessment of the presence or absence of prognostic factors? 

Definitely yes                     Probably yes            Probably no              Definitely no