# Re: st: how to use stcox in time-varying covariate model?

 From rgutierrez@stata.com (Roberto G. Gutierrez, StataCorp) To statalist@hsphsun2.harvard.edu Subject Re: st: how to use stcox in time-varying covariate model? Date Wed, 31 May 2006 16:34:17 -0500

```Feng Lu <lufengs@yahoo.com> asks:

> duration model with time-varying covariates, how do I realize it by stcox?
> Id  time-varying variable year failure    duration
> 1          1.1           2000       0           1
> 1          1.35          2001        1          2
> 2       1.14         2000       0            1
> 2         1.39          2001        0            2
> 2          1.61         2002     0               3
> 2           1.89         2003       0          4
> 3         1.03           2000    0               1
> 3        1.30            2001    0               2
> 3          1.68         2002      1             3
> I read the stata book and found that the example for time-varying covariate
> model is different from mine.
> I tried two ways but both are wrong.
> One way: stset duration, id(id) failure(some variable)

[followed by -stcox-, presumably]

> It seems that this way only used the time-varying variable in the
> first year.

If you were to -stset- the data as you describe, and then call -stcox- with
the time-varying variable as one of your covariates, the time-varying nature
of that variable is indeed automatically accounted for by -stcox-.

Cox regression works via an accounting of those at risk of failure at each
separate instance of time at which a failure occurs.  Since this set (commonly
called a risk set) is indexed by the failure time associated with it, it is
quite easy to account for time-varying covariates -- you simply take the value
of the covariate at that time.  There is no additional complication over what
you would need for time-constant covariates and, as such, there is no

Feng Lu also considered using option -tvc()- and was confused by the output.
The -tvc()- option is not needed with data, such as the above, that has the
time-varying nature of the covariate(s) spelled out in multiple observations
per subject.  Instead, -tvc()- is for use when you want a covariate to
change as a continuous function of time, i.e.

time-varying covariate = (constant covariate in my data) * f(t)

where f(t) is some function of time, with f(t) = t by default and settable
via option -texp()-.

--Bobby
rgutierrez@stata.com
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