Unmasking the unasked : correcting the record about assessor masking as an explanation for effect size differences.

Ainsworth et al.’s paper “Sources of Bias in Outcome Assessment in Randomised Controlled Trials: A Case Study” examines alternative accounts for a large difference in effect size between 2 outcomes in the same intervention evaluation. It argues that the probable explanation relates to masking: Only one outcome measure was administered by those aware of participants’ treatment assignment. This paper shows this conclusion is not substantiated by the evidence: The original paper fails to exclude alternative explanations, and what it takes as positive evidence for the preferred explanation is actually negative. While accepting the importance of masking in randomised controlled trials, this paper concludes that the original question was based on a misconception about effect sizes: Seen correctly as a measure of whole study design, the question of effect size difference between different outcome measures does not need asking