Confirmatory analysis of survival data using left truncation of the life times of primary survivors

In survival analysis, as in other branches of applied statistics, confirmatory analysis is often requested before an initial finding can be cleared of suspicion of being due just to mass significance. This paper points out that the special temporal structure of the classical clinical trial with staggered entry allows re-use of the initial survivors by left-truncating their remaining survival times. This procedure can often save a considerable amount of calendar time. The procedure is illustrated by data from the Danish Breast Cancer Cooperative Group.