# EXERCISE 3.9
# =============

# Read leukemia data:
leukemia=read.table("http://folk.uio.no/borgan/abg-2008/data/leukemia.txt",header=T)

# We will use the survival library, so this has to be loaded.

# We may plot the Nelson-Aalen estimates for the two groups (cf exercise 3.1):
fit=coxph(Surv(time,status)~strata(treat),data=leukemia)
surv=survfit(fit)
plot(surv,fun="cumhaz", mark.time=F,xlim=c(0,25),ylim=c(0,4),
     xlab="Weeks",ylab="Cumulative hazard",lty=1:2)
legend("topleft",c("Placebo","6-MP"),lty=1:2)


# To test if the observed difference is significant, we may use the logrank test:
survdiff(Surv(time,status)~treat,data=leukemia)


# We find that the difference is highly significant.