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Re: st: RE: Interpretation of tvc in stcox


From   Enzo Coviello <[email protected]>
To   [email protected]
Subject   Re: st: RE: Interpretation of tvc in stcox
Date   Thu, 08 May 2008 20:52:24 +0200

Hi,

Thank you for your interest. To answer your questions, yes, the texp(_t) is how the variables
change with time. It is the default so did not need to be included in the stcox statement.

My data is set up so that there are multiple records per individual, with different rows for each
visit. Using the 'id' option in the stset statement, Stata will know how to identify records
within an individual, ie. 360 subjects, 1577 records:
Behavioral variables were measured at each visit (time varying), and were specified with the tvc
option.

I believe that a distinction should be done between time-varying covariates and time-varying effects.
The purpose of the options tvc() and texp() is addressed to model time-varying effects.

In other words you can have time-varying covariates and model hazard ratios that are fixed in the time.
Using tvc() and texp() hazard ratios varying along the time can be modeled. This can be useful if proportional hazards assumption does not hold.
Time-varying effects do not require time-varying covariates in the dataset.

Best wishes

Enzo


Enzo Coviello
Unit� di Epidemiologia e Statistica ASL BARI
Piazza V. Emanuele 14
70054 Giovinazzo (Bari)
Italy
tel./fax +39 080 3357867
mobile +39 347 5016 016
home +39 0883 695055


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