Understanding controlled trials: Randomisation methods: concealment

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Abstract
Randomisation is the best method removing selection bias between two groups of patients.1 However, the process of randomisation can be compromised such that the allocation results in biased groups of patients. A trial which has had its randomisation compromised may apparently show a treatment effect that is entirely due to biased allocation. The results of such a study are more damaging than an explicitly unrandomised study, as bias in the latter is acknowledged and the statistical analysis and subsequent interpretation takes this into account. Changes in clinical management based on a compromised trial may, at best, waste valuable health care resources on a useless treatment; at worst, they may also damage patients' health. The randomisation process must therefore not be compromised. In the past attempts were not generally made to conceal randomisation schedules from investigators …