# EXERCISE 3.1
# =============

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


# We compute Nelson-Aalen estimates and plot them in one figure 
# (using the default "efron" method for handling tied failure times):

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)