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Randomisation methods: concealment

BMJ 1999; 319 doi: https://doi.org/10.1136/bmj.319.7206.375 (Published 07 August 1999) Cite this as: BMJ 1999;319:375
  1. David J Torgerson, senior research fellowa,
  2. Chris Roberts, senior research fellowb
  1. a National Primary Care Research and Development Centre, Centre for Health Economics, University of York, York YO1 5DD
  2. b National Primary Care Research and Development Centre, University of Manchester, Manchester M13 9PL
  1. Correspondence to: Dr Torgerson

    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 …

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