ABSTRACT
In many medical studies, the main outcome variable is the time to the occurrence of a particular event. In a randomized controlled trial of treatment for cancer, for example, surgery, radiation, and chemotherapy might be compared with respect to time from randomization and the start of therapy until death. In this case the event of interest is the death of a patient, but in other situations, it might be remission from a disease, relief from symptoms, or the recurrence of a particular condition. Such observations are generally referred to by the generic term
survival data
even when the endpoint or event being considered is not death but something else. Such data generally require special techniques for their analysis for two main reasons:
• Survival data are generally not symmetrically distributed — they will often appear positively skewed, with a few people surviving a very long time compared with the majority. Thus, assuming a normal distribution will not be reasonable.
