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st: How do you interpret texp commands?


From   "Sudhinaraset, May" <msudhina@jhsph.edu>
To   "statalist@hsphsun2.harvard.edu" <statalist@hsphsun2.harvard.edu>
Subject   st: How do you interpret texp commands?
Date   Tue, 12 Apr 2011 10:38:38 -0400

Hello everyone,

I am modeling time varying effects using stcox. I am confused as to 1) how to specify texp function, and 2) how to interpret output for tvc and texp. 

For example, my command is: stcox ruralnative ruraltourban urbantourban, tvc(ruralnative ruraltourban urbantourban) texp((_t-16)) cluster(psu_shang)

What is the difference between specifying texp(_t-16) and texp(_t<16)? I am assuming that the first is at a single point in time, while the second is all data before 16. Is this correct? 

Let's say that I use texp(_t<16): is this like introducing a spline term with changing time? 

Also, for interpretations, are TVC coefficients the hazard ratio BEFORE age 16, whereas the main coefficient the hazard ratio AFTER age 16? Not sure how to report this in a paper.

Any help would be appreciated. Thanks!
May 

____________________________________________
May Sudhinaraset
Ph.D. Candidate
Department of Population, Family, and Reproductive Health
Bloomberg School of Public Health
Johns Hopkins University
Email: msudhina@jhsph.edu
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