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st: R: How to avoid uncontrolled confounding in Cox regression analysis


From   "Carlo Lazzaro" <carlo.lazzaro@tin.it>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: R: How to avoid uncontrolled confounding in Cox regression analysis
Date   Tue, 8 Sep 2009 12:46:37 +0200

Roland Andersson wrote:

<..., as there is probably violation of
proportional hazard due to the difference in age.>

Violation of proportional hazard assumption can be easily detected via
Schoenfeld residuals (please, type - help stcox - in Stata 9/2 SE).  

<I wonder if a stratified analysis would be better (with appropriate
age-intervals as strata)...>

In my opinion, a stratified analysis by age would be fine if, accordingly to
the e xistent epidemiological literature on this topic (that, unfortunately,
I do not know)differently aged people experience different hazard functions.

Two other comments come to my mind thinking over your research endeavour:

- provided that it is consistent with the existing epidemiological
literature and with the clinical evidence, wouldn't it be wise to include a
squared age terms in the RHS of Cox regression to detect any turning point
of the failure process?
- would it be interesting to consider categorical variable to tag the
different clinical subtypes of appendicitis?  

Kind Regards,
Carlo
-----Messaggio originale-----
Da: owner-statalist@hsphsun2.harvard.edu
[mailto:owner-statalist@hsphsun2.harvard.edu] Per conto di roland andersson
Inviato: martedì 8 settembre 2009 10.59
A: statalist@hsphsun2.harvard.edu
Oggetto: st: How to avoid uncontrolled confounding in Coxregression analysis

I have a large national dataset and plan to analyse socioeconomic
differences in risk of having appendicitis. Persons born after 1949 to
end of 1990 and alive at 1/1/1991 (n=4.500.000) are included in the
dataset. The dataset contain the date when a person was operated for
appendicitis between 1/1/1991 and 31/12/2003 and date of deaths.

Previous collaborators have used SPSS and made Coxregression with time
from 1/1/1991 to the appendicitisdiagnosis or censoring as
analysistime, and have entered age at operation as covariates to
control for the differnces in age.

I am not content with the results and think that there may be
uncontrolled confounding from age, as there is probably violation of
proportional hazard due to the difference in age.

I wonder if a stratified analysis would be better (with appropriate
age-intervals as strata) or if I can use age as analysistime with the
persons coming under observation at different ages.

Is this a correct form to make stset:

stset dateexit [fweight = count], failure(app) enter(time dateenter)
exit(time dateexit) origin(time datebirth) scale(365.25)

with dateenter=1/1/1991

     failure event:  app != 0 & app < .
obs. time interval:  (origin, dateexit]
 enter on or after:  time dateenter
 exit on or before:  time dateexit
    t for analysis:  (time-origin)/365.25
            origin:  time datebirth
            weight:  [fweight=count]

I would appreciate your comments.

Roland Andersson, MD PhD
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