coxme proportional hazard assumption

Question

I am running mixed effect Cox models using the coxme function {coxme} in R, and I would like to check the assumption of proportional hazard.

I know that the PH assumption can be verified with the cox.zph function {survival} on cox.ph model.

However, I cannot find the equivalent for coxme models.

In 2015 a similar question has been posted here, but had no answer.

my questions are: 1) how to test PH assumption on mixed effect cox model coxme? 2) if there is no equivalent of the cox.zph for coxme models, is it valid for publication in scientific article to run mixed effect coxme model but test the PH assumption on a cox.ph model identical to the coxme model but without random effect?

Thanks in advance for your answers. Regards

Answer 1

You can use frailty option in coxph function. Let's say, your random effect variable is B, your fixed effect variable is A. Then you fit your model as below

myfit <- coxph( Surv(Time, Censor) ~ A + frailty(B) , data = mydata )

Now, you can use cox.zph(myfit) to test the proportional hazard assumption.

Answer 2

I don't have enough reputation to comment, but I don't think using the frailty option in the coxph function will work. In the cox.zph documentation, it says:

Random effects terms such a frailty or random effects in a coxme model are not checked for proportional hazards, rather they are treated as a fixed offset in model.

Thus, it's not taking the random effects into account when testing the proportional hazards assumption.