Prognostic factors in clinical trials.

Prognostic factors define the study population, help formulate the study objectives, and influence the treatment strategies. They must be accounted for in the study analysis to obtain valid estimates of the treatment differences and to evaluate results across studies. The causal relationship between a prognostic factor and the study endpoint can only be established through prospective randomized study designs. Potential factors discovered through retrospective analysis must be verified to establish their validity. Using such factors prematurely to select patient population and treatment strategy for a new study will not establish the validity of the potentially important factor.