SEQUENTIAL MONITORING OF SURVIVAL-DATA WITH THE WILCOXON STATISTIC

When a spending function is used in sequential data monitoring of a cl inical trial, it is important to know the information fraction at the times of interim analysis. In a maximum duration designed study, the i nformation fraction is unknown when data are monitored, and it has to be estimated. The modified Wilcoxon statistic developed by Peto and Pe to and modified by Prentice is often used to compare two survival curv es in a clinical trial. We give guidelines for estimating the informat ion fraction in a maximum duration trial when this statistic is employ ed. When there is a relatively low event rate or the survival time is approximately exponential, the information fraction for the Peto-Peto- Prentice Wilcoxon statistic is very close to that of the popular logra nk statistic. In other cases, it would be helpful to estimate the info rmation fraction as a function of elapsed calendar time. We discuss bo th group sequential and continuous monitoring.