# NOT RUN {
  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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