Practical exercise 3
In this exercise we will use the Kaplan-Meier estimator to study survival for the melanoma patients.
As described in practical exercise 1, you may read the data into R by the command:
melanoma=read.table("http://www.uio.no/studier/emner/matnat/math/STK4080/h12/melanoma.txt", header=T)
We will compute Kaplan-Meier estimates for the mortality from melanoma treating death from other causes as censoring. If we consider all patients, this may be performed by the command (you need to load the survival-library)
fit.mel0=survfit(Surv(lifetime,status==1)~1,data=melanoma,conf.type="plain")
We may plot the Kaplan-Meier estimate with standard confidence limits by
plot(fit.mel0, mark.time=F, xlab="Years after operation")
The following command gives a summary of the results:
summary(fit.mel0)
a) Make Kaplan-Meier plots for males and females and interpret the plots. Determine the lower quartile with 95% confidence limits for males and females using the output from the summary-command. (Note that the lower quartile corresponds to 75% survival probability.)
b) Make Kaplan-Meier plots for patients with ulceration present and absent and interpret the results. Is it possible to estimate the lower quartile for both ulceration groups? Estimate the lower quartile with confidence limits if possible.
c) Make Kaplan-Meier plots for the three thickness groups 0-