Blinding And Bias In Randomized Controlled Trials: When To Measure The Effectiveness Of Blinding

EDITOR–A recent letter by Hastings‐Ison and Graham1 considered three randomized controlled trials (RCTs) that report the outcome of botulinum neurotoxin A (BoNT‐A) injections in children with cerebral palsy (CP), and asserted that they were not effectively blinded. This assertation is made because, at follow‐up, parents and researchers were able to identify which group (BoNT‐A or placebo) the child was allocated to. However, a question such as ‘Do you think your child received BoNT‐A or placebo?’ can only be used to assess the effectiveness of blinding if it is answered before any effect due to the treatment occurs. If the same question is asked after the treatment has taken effect, then it becomes an outcome measure – a measure of the parent's expectation of the intervention's success.