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, s_w², and the variance of true cluster means, s_c².1 We described how such a study could be analysed using the sample cluster means. The variance of such means would be s_c²+s_w²/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 s_w², and between subjects (s_{b …}