Statistics notes

Treatment allocation in controlled trials: why randomise?

BMJ 1999; 318 doi: http://dx.doi.org/10.1136/bmj.318.7192.1209 (Published 1 May 1999)
Cite this as: BMJ 1999;318:1209.1

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  1. Douglas G Altman, professor of statistics in medicinea,
  2. J Martin Bland, professor of medical statisticsb
  1. aICRF Medical Statistics Group, Centre for Statistics in Medicine, Institute of Health Sciences, Oxford OX3 7LF
  2. bDepartment of Public Health Sciences, St George's Hospital Medical School, London SW17 0RE
  1. Correspondence to: Professor Altman.

    Since 1991 the BMJ has had a policy of not publishing trials that have not been properly randomised, except in rare cases where this can be justified.1 Why?

    The simplest approach to evaluating a new treatment is to compare a single group of patients given the new treatment with a group previously treated with an alternative treatment. Usually such studies compare two consecutive series of patients in the same hospital(s). This approach is seriously flawed. Problems will arise from the mixture of retrospective and prospective studies, and we can never satisfactorily eliminate possible biases due to other factors (apart from treatment) that may have changed over time. Sacks et al compared trials of the same treatments in which randomised or historical controls were used and found a consistent tendency for historically controlled trials to yield more optimistic results than randomised trials.2 The use of historical controls can be justified only in tightly controlled situations of relatively rare conditions, such …

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