# PRACTICAL EXERCISE 4 # ==================== # Read leukemia data: leukemia=read.table("http://folk.uio.no/borgan/abg-2008/data/leukemia.txt",header=T) # We use the survival library, so this has to be loaded. # Question a # ---------- # We compute the estimates of the survival function for the placebo group with the # options "kaplan-meier", "fleming-harrington", and "fh2" (the first is default) surv.km=survfit(Surv(time,status)~1,data=leukemia,subset=(treat==1),type="kaplan-meier") surv.fl=survfit(Surv(time,status)~1,data=leukemia,subset=(treat==1),type="fleming-harrington") surv.fh2=survfit(Surv(time,status)~1,data=leukemia,subset=(treat==1),type="fh2") # We then list the estimates: cbind(surv.km\$time, surv.km\$surv, surv.fl\$surv, surv.fh2 \$surv) # We see that estimates are close at early times when there is a fair number of individuals at risk. # The difference is larger at later times when there are only a few individuals at risk. # The Kaplan-Meier estimator is smallest, and the Flemming-Harrington is largest. # Question b # ---------- # We then compute the two standard error estimates: # * error="greenwood" in combination with type="kaplan-meier" (default) # * error="tsiatis" in combination with type="fh2" surv.gw=survfit(Surv(time,status)~1,data=leukemia,subset=(treat==1), type="kaplan-meier",error="greenwood",conf.type="none") surv.ts=survfit(Surv(time,status)~1,data=leukemia,subset=(treat==1), type="fh2",error="tsiatis", conf.type="none") # We may use the summary command to see the estimated standard errors summary(surv.gw) summary(surv.ts) # The see that the standard errors are quite close