In addition to considerations of blinding, which are similar to those in randomised trials, it is important to consider whether exposure was measured in a similar and unbiased way in the groups being compared. So, for example, in addition to concerns about bias due to confounders in cohort and case control studies of the effects of post-menopausal hormone replacement therapy, investigators and reviewers must ensure that use of hormones was measured in an unbiased way. In summary, a great deal of judgement is necessary in assessing the validity of observational studies. Judgement is also needed when the validity of randomised trials is assessed, but the nature of observational studies makes them even more difficult to critically appraise. This requires a thorough understanding of both the problem that is the focus of the review and methodological considerations.
Caution is needed.