Article first published online: 26 JUN 2012
Copyright © 2012 John Wiley & Sons, Ltd.
Statistics in Medicine
Special Issue: Special Issue in Honor of Jerome Cornfield on the Centennial of His Birth
Volume 31, Issue 24, pages 2822–2832, 30 October 2012
How to Cite
Caille, A., Leyrat, C. and Giraudeau, B. (2012), Dichotomizing a continuous outcome in cluster randomized trials: impact on power. Statist. Med., 31: 2822–2832. doi: 10.1002/sim.5409
- Issue published online: 5 OCT 2012
- Article first published online: 26 JUN 2012
- Manuscript Accepted: 15 MAR 2012
- Manuscript Received: 29 JUN 2011
- cluster randomized trial;
- intraclass correlation coefficient;
In cluster randomized trials (CRTs), clusters of individuals are randomized rather than the individuals themselves. For such trials, power depends in part on the degree of similarity among responses within a cluster, which is quantified by the intaclass correlation coefficient (ICC). Thus, for a fixed sample size, power decreases with increasing ICC. In reliability studies with two observers, dichotomizing a continuous outcome variable has been shown to reduce the ICC. We checked (by a simulation study) that this property still applies to CRTs, in which cluster sizes are variable and usually greater than in reliability studies and observations (within clusters) are exchangeable. Then, in a CRT, dichotomizing a continuous outcome actually induces two antagonistic effects: decreased power because of loss of information and increased power induced by attenuation of the ICC. Therefore, we aimed to assess the impact of dichotomizing a continuous outcome on power in a CRT. We derived an analytical formula for power based on a generalized estimating equation approach after dichotomizing a continuous outcome. This theoretical result was obtained by considering equal cluster sizes, and we then assessed its accuracy (by a simulation study) in the more realistic situation of varying cluster sizes. We showed that dichotomization is associated with decreased power: attenuation of the ICC does not compensate for the loss of power induced by loss of information. Loss of power is reduced with increased initial continuous-outcome ICC and/or prevalence of success for the dichotomized outcome approaching 50%. Copyright © 2012 John Wiley & Sons, Ltd.