The presence of a non-zero intracluster correlation coefficient in cluster randomized trial data has well-known statistical implications for trial design, in particular, inflating the required sample size for given specifications. However, problems in recruitment are common in such trials and there may be different costs of recruitment resulting from different recruitment strategies. Examples of how such differences arise are taken from cluster randomized trials and more intuitive methods of describing clustering are summarized which, in conjunction with such cost issues, may provide a framework that enables triallists to consider explicitly the trade-off between power and cost that is often inherent in such trials.
Copyright 2002 John Wiley & Sons, Ltd.