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Re: st: poisson vs cox


From   Enzo Coviello <[email protected]>
To   [email protected], [email protected], [email protected]
Subject   Re: st: poisson vs cox
Date   Sat, 02 Dec 2006 11:15:12 +0100

Hi Raoul,

using age as time axis the effect of age at diagnosis should not be clearly estimated by the Cox model.
So, any effect of this covariate in the derived Poisson model could be a residual effect caused by the aggregated nature of data.

Changing time axis and introducing the appropriate regressor for it in Poisson model could be a useful check of this hypothesis.

Best
Enzo


Hi,

I am running a poisson regression, but if I run a cox regression and
use the same covariates, then some of the rate ratios clearly differ
between the models. I always thought that a cox regression should give
approx. the same results as a poisson regression. What could be the
reason for this?

In more detail:

I first stset with age as time:
.stset dox, fail(fail) enter(doe) origin(dob) id(id)

then I used stsplit to create age and calender period bands and used
poisson regression with additional covariates (sex, age at diagnosis,
treatment etc)..

then, as a check and out of curiousity I ran a cox regression model,
but the rate ratios for treatment and in particular for age at
diagnosis differ between the models.
Thanks for your help.

Raoul
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