Letter

Cluster randomised trials

BMJ 1999; 318 doi: 10.1136/bmj.318.7193.1286b (Published 8 May 1999)
Cite this as: BMJ 1999;318:1286.3

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Standardised approach to analysing and reporting these trials is misguided

  1. Nick Freemantle, Senior research fellow,
  2. John Wood, Principal statistician
  1. Medicines Evaluation Group, Centre for Health Economics, University of York, York YO10 5DD
  2. Statistical Sciences, SmithKline Beecham Pharmaceuticals, Harlow, Essex CM19 5AW
  3. Health Services Research Unit, University of Aberdeen, Aberdeen AB25 2ZD

    EDITOR—In their editorial on cluster randomised trials Campbell and Grimshaw correctly identify the importance of the appropriate choice of the unit of analysis.1 Unfortunately, they display a disappointingly poor grasp of the basis of estimation, and the specific recommendations that they make are inappropriate.

    The authors' plea for a standardised approach to the analysis and reporting of cluster randomised trials is misguided. The framework that they describe has the patient as the principal unit of the experiment, with the correlation between patients in the same cluster having to be taken into account in the analysis. Following the recommendations in the editorial may, however, make investigators prey to the same errors that the authors are counselling them to guard against.

    Although some trials fall naturally into the framework that the authors describe, others do not. It is not helpful in their own example of an educational intervention to implement a …

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