Some exact tests for comparing survival curves in the presence of unequal right censoring

Abstract

It is shown that through the use of conditional tests after artificial censoring one may produce analogues of standard nonparametric tests, such as the log rank and generalized Wilcoxon, that are exact even in the presence of unequal right censoring where standard permutation tests fail. The artificial censoring leads to a loss in power, but in important special cases, as for example in the study of rare diseases, this can be minor. The power loss is investigated through a study of the asymptotic efficiency of the proposed tests. An application is made to test for a suspected carcinogen.