Characterizing onset in psychopharmacological clinical trials.

In addition to describing treatment efficacy in terms of changes in rating scale scores, the distributions of time to occurrence of major clinical events such as onset and response are clinically important information. Issues in the design, conduct and analysis of clinical trials in which the time to onset of effect or time to response is to be characterized are discussed. A criterion must be defined to signal that the clinical event has occurred. Onset properties are given in terms of (1) the probability of obtaining onset and (2) for patients who obtain onset, the distribution of time to onset. A statistical model and methods to estimate parameters and compare onset times of treatments are described. A simple formula that can be used to aid in clinical decision making as to when to alter treatment if onset has not yet occurred is presented.