An analysis for transient states with application to tumor shrinkage.

The evaluation of therapies for chronic diseases is often based on the frequency and/or the duration of improvement. Treated separately, these endpoints may give contradictory impressions of the efficacy of the therapy. We propose a more unified method of summarizing improvement-related data--the probability of being in response, i.e., improved, as a function of time. Although improvement is not the only endpoint considered in most trials and this function will not always provide a clear answer to the question of which treatment has better improvement-related characteristics, it does combine the information on several endpoints usually considered separately into a single easily interpreted item. This function is estimated using the method of maximum likelihood on a distribution-free stochastic model of times to improvement and failure. Censored observations are taken into account. A detailed example using data from a cancer clinical trial is presented.