Intended for healthcare professionals


Applying results of randomised trials to patients

BMJ 1998; 317 doi: (Published 22 August 1998) Cite this as: BMJ 1998;317:537

N of 1 trials are needed

  1. Stephen Senn, Professor of pharmaceutical and health statistics. (
  1. Department of Epidemiology and Public Health, Department of Statistical Science, University College London, London WC1E 6BT
  2. Department of Clinical Epidemiology, McMaster University, Hamilton, Ontario, Canada L8N 3Z5
  3. Division of Respiratory Medicine, University of Toronto, Toronto, Ontario, Canada

    EDITOR—Guyatt et al's proposal for analysing randomised trials1 is misguided, flies in the face of elementary statistical theory, and should be resisted. There are three obvious sources of variability in clinical trials.Firstly, pure differences occur between patients:some are more seriously ill than others. Secondly, there is variability within patients: even given the same treatment they, or their measurements, may vary from time to time. Thirdly, some patients may react more favourably to a given treatment than other patients. The parallel group trial does not and cannot distinguish between the three types of variability unless we can find meaningful ways of classifying subgroups.2The standard crossover trial will distinguish between the first type of variability and …

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