Sample size determination for clinical trials with co-primary outcomes: exponential event times.

Clinical trials with event-time outcomes as co-primary contrasts are common in many areas such as infectious disease, oncology, and cardiovascular disease. We discuss methods for calculating the sample size for randomized superiority clinical trials with two correlated time-to-event outcomes as co-primary contrasts when the time-to-event outcomes are exponentially distributed. The approach is simple and easily applied in practice.