if (FALSE) library(survival)
data(pbc)
pbc1 <- pbc
pbc1$trt[pbc1$trt == -9] <- NA
pbc1$copper[pbc1$copper == -9] <- NA
calogrank(pbc1$time, pbc1$status, pbc1$trt, pbc1[,c("copper")])
calogrank(pbc1$time, pbc1$status, pbc1$trt,
pbc1[,c("protime", "copper")])
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