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Comparability of Randomised Groups

Douglas G. Altman
Journal of the Royal Statistical Society. Series D (The Statistician)
Vol. 34, No. 1, Statistics in Health (1985), pp. 125-136
Published by: Wiley for the Royal Statistical Society
DOI: 10.2307/2987510
Stable URL: http://www.jstor.org/stable/2987510
Page Count: 12
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Comparability of Randomised Groups
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Abstract

Randomised allocation in a clinical trial does not guarantee that the treatment groups are comparable with respect to baseline characteristics. It is common for differences between treatment groups to be assessed by significance tests but such tests only assess the correctness of the randomisation, not whether any observed imbalances between the groups might have affected the results of the trial. In particular, it is quite unjustified to conclude that variables that are not significantly differently distributed between groups cannot have affected the results of the trial. The possible effect of imbalance in a prognostic factor is considered, and it is shown that non-significant imbalances can exert a strong influence on the observed result of the trial, even when the risk associated with the factor is not all that great. It is suggested that comparability should be assessed partly on the basis of clinical knowledge and that it is wise to investigate whether any imbalances between the groups could have had an affect by reanalysing the data taking such factors into account. The implications for trial design are considered briefly.

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