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

From   Jocelyn Andrel <>
Subject   st: Just what are -tvc- and -texp- doing in -stcox-?
Date   Fri, 31 Jul 2009 10:41:52 -0400


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)


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,

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