8.8 In section 1.3, data gathered from a multicenter trial of patients in three groups (ALL, AML low-risk, and AML high-risk) was followed after transplantation until relapse, death, or end of study. One of the covariates recorded in the data set is the waiting time to transplant (in days). Use Breslow's method for handling ties in the following.

(a) You are asked to categorize patients into high- or low-risk groups for disease-free survival based on the waiting time to transplant variable for the ALL group.

(b) Analyze the data using waiting time to transplant as a categorized variable using the cut point obtained in (a). Give the parameter estimate, standard error, and relative risk of the high-risk group compared to the low-risk group for the ALL group.

(c) Analyze the data using waiting time to transplant as a continuous variable. Give the parameter estimate, standard error, and relative risk for waiting time to transplant for the ALL group. Compare with answer in (b).

use the "KMsurv" package in R to access the "btrial" data set