Planning a reproducibility study: how many subjects and how many replicates per subject for an expected width of the 95 per cent confidence interval of the intraclass correlation coefficient

Reproducibility of a quantitative outcome is usually assessed by means of t he intraclass correlation coefficient (ICC). When we are interested in asse ssing reproducibility from only one sample, we suggest that the study be pl anned with regards to the expected width of the 95 per cent confidence inte rval of the ICC. An approximation of this latter width is derived, which al lows appraisal of the influence of n the number of subjects and p the numbe r of replicates. Through simulation studies, the approximation is shown to be of good accuracy and can therefore be used reliably. Optimal designs are also discussed such as the optimal distribution between the number of subj ects and the number of replicates per subject for a fixed total number of m easures. Copyright (C) 2001 John Wiley & Sons, Ltd.