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EDUCATION AND DEBATE: Sally M Kerry and J Martin Bland Statistics notes: Sample size in cluster randomisation BMJ 1998; 316: 549 [Abstract] [Full text]

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Intracluster correlation underestimated?

Johannes C van der Wouden   (26 April 1998)

Re: Intracluster correlation underestimated?

J Martin Bland, Sally Kerry   (8 June 1998)

Intracluster correlation underestimated? 26 April 1998
Johannes C van der Wouden, research coordinator Dept of General Practice, Erasmus University Rotteram, The Netherlands Send response to journal: Re: Intracluster correlation underestimated?	Sir, I welcome the contribution of Kerry and Bland on cluster randomization (BMJ 1998; 316: 549). As long as our world contains more patients than doctors, this issue should be taken into account when planning a study. The example they present, however, may be somewhat misleading. The proposed study addresses the effect of two interventions on cholesterol level. One is an intensive dietary intervention by practice nurses, the other is usual general practice care. The data they use to estimate clustering are on cholesterol levels from a thrombosis prevention trial. It is unclear whether these are pre- or post-intervention data, but in both cases I doubt whether the magnitude of clustering of cholesterol concentrations in this study is a reliable estimate for the proposed study. I would not be surprised when the clustering in the practice nurse intervention would turn out to be much higher, as this may be much more affected by personal factors. Johannes C van der Wouden Department of General Practice Erasmus University Rotterdam PO Box 3000 1738 DR Rotterdam The Netherlands email: vanderwouden@hag.fgg.eur.nl fax + 31 10 436 07 17 tel + 31 10 408 76 11
Re: Intracluster correlation underestimated? 8 June 1998
J Martin Bland, Prof of Medical Statistics St. George's Hospital Medical School, Sally Kerry Send response to journal: Re: Re: Intracluster correlation underestimated?	Dr van der Wouden makes a good point. In any sample size calculation, care should be taken to choose appropriate values for the variances. Such calculations inevitably involve some guess-work, however, and are always approximate. It is a good idea to consider the implications of greater variability than anticipated. We should also err on the side of caution. Researchers are often criticised for taking a sample which is too small, seldom for taking one which is too large. It may well be that in this case variation between clusters, and hence the intracluster correlation coefficient, would be increased by the presence of a different treatment provider in each cluster. In fact, in a study with a similar intervention, as yet unpublished, we obtained values very like those cited. We need more information about the variances and intracluster correlations found in different types of cluster randomised trials, to help future trials to be better designed.