Importance of Blinding in Randomized Trials

To the Editor: In their Commentary, Drs Psaty and Prentice¹ added to the available evidence that lack of blinding in randomized trials favors the intervention, even with studies that have what are considered hard end points. We agree about the importance of investigator blinding and would add a few observations and some cautions to their statements.

Psaty and Prentice state that nondifferential errors between treatment groups “may not systematically bias . . . summary measures although precision will be reduced.” However, as a general principle nondifferential errors between treatment groups could systematically bias summary measures if the balance between treatment groups resulting from randomization were disrupted. For example, an imbalance could be created between the groups if patients with lack of, or unblinded, adjudication in one group were different from their counterparts in the other group in key prognostic variables.