[R] A question on time-dependent covariates in the Cox model.

[Marianne dk]

I have a dataset with

event=death
time (from medical examination until death/censoring)
dose (given at examination time)

Two groups are considered, a non-exposed group (dose=0), an exposed group 
(dose between 5 and 60).

For some reason there is a theory of the dose increasing its effect over 
time (however it was only given (and measured) once = at the time of 
examination).

I tested a model:

coxph(Surv(time,dod)~dose + dose:time)

Previously I tested the model in SAS:

proc phreg data=test;
	model time*dod(0)=dose dosetime /rl ties=efron;
	dosetime=time*dose;
	run;

Without the interaction terms I get the same results for the two models. By 
including the interaction terms I do not. The model in R gives a negative 
coefficient for the interaction term which is expected to be positive (and 
is so in SAS). The LRTs are also completely different.

TWO QUESTIONS:

1) Is it reasonable to bring in an interaction term when dose is only 
measured once?

2) If yes, can anyone give a hint on explaining the difference between the 
models in R and SAS?

Thanx in advance,
marianne

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