st: Just what are -tvc- and -texp- doing in -stcox-?

[Jocelyn Andrel <Jocelyn.Andrel@mail.jci.tju.edu>]

Hi,

I'm having trouble with -stcox-. I need to produce predicted survival 
curves, but my model has a time-dependent covariate, so I can't use 
-tvc- and -texp-, I have to use -stspilt-, then -stcox-. Which would be 
fine, except my unsplit and split model simply don't match. Here's the 
background: the data is time to death from a cancer, the time-dependent 
covariate is radiation after surgery. Some people had it, some didn't, 
so the radiation variable is dichotomous (0/1) for radiation, then the 
time of radiation is there for those who had it, and missing for those 
who didn't.

I really have two questions:
1. How do -tvc- and -texp- work?
2. What's wrong with this code?

My code looks like:

stset timetoevent, failures(censor) id(id)
stcox age, tvc(rad) texp(timeofrad)

or

stsplit at(failures)
gen radtvc = rad*(_t > timeofrad)
stcox age radtvc

I think the issue centers around the timeofrad variable.  It's the time 
to radiation, and it's different for each patient that had it or missing 
for those who didn't.  If I replace it with a constant, the two 
different versions of code match perfectly. (I've also tried replacing 
the missing values in timeofrad with zero, to no avail.)

Any insight would be appreciated. 

Thanks in advance,
Jocelyn




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