The run-in period in clinical trials. The effect of misclassification on efficiency

This article considers the effect of misclassification of potential participants during the run-in period preceding randomization in a clinical trial. We present a simple mathematical model of adherence that allows for misclassification. Simulations based on this model assess the impact of a run-in period on statistical power. The run-in period is most effective when there is a high proportion of poor adherers and a low rate of misclassification. In situations with either a high degree of adherence or substantial misclassification, the run-in period may reduce the efficiency of the trial, particularly when the cost of recruitment is high. We also discuss the early adherence in two recently conducted trials that had no run-in periods in order to estimate the extent of misclassification and consequent effect on power that would have been observed had a run-in occurred.