Good study design involves minimising all possible sources of bias. Two important sources of bias arise through failure to mask (ie, conceal), first, the randomisation process and, second, the treatments after randomisation. Allocation concealment is the term used to describe the procedure for protecting the randomisation process so that the treatment to be allocated is not known before the patient is entered into the study. Blinding relates to the masking of the treatments after randomisation — from the patient, the investigator or the outcomes assessor. Without exception, allocation concealment is achievable in all randomised clinical trials. In contrast, it is not always possible to blind people to study treatments received. The CONSORT statement strongly encourages detailed reporting of the allocation concealment process and the measures taken to preserve blinding (Box 1).1
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