Hello everyone, Many thanks to Terry Therneau for giving me a solution for my previous problem re coxme function. Now, I am using a bigger dataset to fit the same model; random treatment effect nested within centre. I used the command coxme(Surv(time, status) ~ factor(treat), data=data1, random = ~1 | centre/treat) But as soon as I entered this command, R dies with an error message "R for windows GUI front-end has encountered a problem and need to close ". I am using R in Windows, R version is 2.6.0. Has anyone ever had this type of error? Thanks for any help. Ruwanthi