Journal
STATISTICS IN MEDICINE
Volume 23, Issue 8, Pages 1195-1214Publisher
JOHN WILEY & SONS LTD
DOI: 10.1002/sim.1721
Keywords
intracluster correlation coefficient; cluster randomized trials; sample size; power; Bayesian methods; study design
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Funding
- Medical Research Council [G0200391] Funding Source: Medline
- MRC [G0200391] Funding Source: UKRI
- Medical Research Council [G0200391] Funding Source: researchfish
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The sample size required for a cluster randomized trial depends on the magnitude of the intracluster correlation coefficient (ICC). The usual sample size calculation makes no allowance for the fact that the ICC is not known precisely in advance. We develop methods which allow for the uncertainty in a previously observed ICC, using a variety of distributional assumptions. Distributions for the power are derived, reflecting this uncertainty. Further, the observed ICC in a future study will not equal its true value, and we consider the impact of this on power. We implement calculations within a Bayesian simulation approach, and provide one simplification that can be performed using simple simulation within spreadsheet software. In our examples, recognizing the uncertainty in a previous ICC estimate decreases expected power, especially when the power calculated naively from the ICC estimate is high. To protect against the possibility of low power, sample sizes may need to be very substantially increased. Recognizing the variability in the future observed ICC has little effect if prior uncertainty has already been taken into account. We show how our method can be extended to the case in which multiple prior ICC estimates are available. The methods presented in this paper can be used by applied researchers to protect against loss of power, or to choose a design which reduces the impact of uncertainty in the ICC. Copyright (C) 2004 John Wiley Sons, Ltd.
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