The internal validity of a study is defined as the extent to which the observed difference in outcomes between the two comparison groups can be attributed to the intervention rather than other factors. The biggest advantage of randomised controlled trials compared with cohort studies is that the random allocation process enhances the internal validity of a study by minimising selection bias and confounding.6
This paper relies on the definitions provided by CONSORT (box 1).7
Allocation by chance in a randomised controlled trial should mean that the groups being compared are similar in terms of both measured and unmeasured baseline factors.8
This is not so in cohort studies, and therefore cohort studies are vulnerable to selection bias. In cohort studies, factors that determined whether a person received the intervention could result in the groups differing in factors related to the outcome, either because people were preferentially selected to receive one treatment or because of choices that they made. These baseline differences in prognosis could confound the assessment of the effect of the intervention.
Cohort studies can use diverse populations
In cohort studies care must be taken to minimise, assess, and deal with selection bias. A comprehensive approach is needed that includes the selection of appropriate comparison groups, the identification and assessment of the comparability of potential confounders between those comparison groups, and the use of sophisticated statistical techniques in the analysis.