- Sally M Kerry, statisticiana,
- J Martin Bland, professor of medical statisticsb
- a Division of General Practice and Primary Care, St George's Hospital Medical School, London SW17 0RE
- b Department of Public Health Sciences
- Correspondence to: Mrs Kerry
We have described the calculation of sample size when subjects are randomised in groups or clusters in terms of two variances—the variance of observations taken from individuals in the same cluster, sw2, and the variance of true cluster means, sc2.1 We described how such a study could be analysed using the sample cluster means. The variance of such means would be sc2+sw2/m, where m is the number of subjects in a cluster. We used this to estimate the sample size needed for a cluster randomised trial.
This sum of two components of variance is analogous to what happens with measurement error, where we have the variance within the subject, also denoted by sw2, and between subjects (sb …