st: discrete time-varying covariate in cox models

["Daniel O. Koralek" <dkoralek@unc.edu>]

Hello,

I have a scenario where I have a time-varying covariate, the number of  
screens that an individual has undergone, at up to 6 time points  
during follow-up.  To simplify matters, let's say that I have a single  
covariate that I am modeling plus my screening variable, so the data  
set includes (and I am using age as the underlying time metric):
id entryage exitage iscase (my failure variable) x (my exposure)  
screenage0 - screenage5 (includes the ages at which the potential  
screening events occurred) and screen0 - screen5 (which simply is the  
number of screens that have been experienced up to and including  
screens that may have occurred at the corresponding screenage (i.e.  
screen2 corresponds to screenage2).

so, the stset looks like:
stset exitage, failure(iscase==1) enter(time entryage) exit(time  
exitage) scale(1)

Now, what I would like to do is simply control for a single screen  
variable that equals the number of screens that occurred up to the  
analysis time.  THe examples that I have seen using stsplit seem to  
only use a single change (in this scenario, up to a certain point  
screen =0 and after screen = 1), not where multiple changes can occur.

Hopefully this makes sense.  Any pointers or references would be  
greatly appreciated.

Thanks for your time.

Sincerely,

Daniel O. Koralek
Department of Epidemiology/Lineberger Comprehensive Cancer Center
The University of North Carolina at Chapel Hill
Chapel Hill, NC  27599-7435





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