  We analyse an issue when comparing survival curves between two subgroups. We
show that there is a direct relationship between estimates of subgroups'
survival at a time point and positive and negative predictive values in the
binary classification settings. Our findings present a case where current
methods of comparing survival curves between subgroups may be misleading. We
think that this ought to be taken into account during the validation of
prognostic diagnostic tests that predict two prognostic subgroups for a given
disease or treatment, when the validation data set consists of censored data.
