Commentary: Measuring the success of blinding in RCTs: don’t, must, can’t or needn’t? International Journal of Epidemiology 36(3)

Elsewhere in this issue, Asbjorn Hrobjartsson and his colleagues at the Nordic Cochrane Center present an analysis of the success of blinding in a random sample of entries in the Cochrane Central Register of Controlled Trials,1 adding to the recent analyses of ‘blinded’ trials from ‘top journals’ published by Dean Fergusson and his colleagues,2 and of trials identified through MEDLINE, Cochrane registries, and ‘high-impact-factor journals’ by Isabelle Boutron and her colleagues3 (No article appeared in all three reviews, and only three articles appeared in two reviews.) The first two teams found that only 2 and 8% of the ‘blinded’ trials they unearthed had tested for blindness, respectively. Furthermore, in all three studies, these tests concluded that blinding was successful half of the time or less. Thus, the vast majority of ‘blinded’ trials don't report testing whether study participants or those who care for or assess them can identify which treatment was given.

Must blinded trials test for blindness? The authors of these three reviews all favour improved testing and reporting, but none outright insist that it must be measured in every blinded trial. Moreover, the CONSORT checklist of items to include when reporting a randomized trial is being modified to remove the sentence: ‘If done, how the success of blinding was evaluated.’4 Thus, none of these sources insist that all blinded trials must test for blindness.