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st: RE: Why don't my IRs and Cox HRs echo each other?

From   "Kieran McCaul" <>
To   <>
Subject   st: RE: Why don't my IRs and Cox HRs echo each other?
Date   Thu, 7 May 2009 07:56:51 +0800

I'd check the proportionality assumption for the Cox models.  
Also use Kaplan-Meier to estimate hazard curves for each of the
variables to get an idea of when the deaths are occurring.

The Cox model is using the rank order of the deaths, so you could have
two groups with similar mortality rates overall, but if the deaths in
one group tended to occur earlier in the course of follow-up then in the
other, the Cox model would pick this as the high risk group. 

Kieran McCaul MPH PhD
WA Centre for Health & Ageing (M573)
University of Western Australia
Level 6, Ainslie House
48 Murray St
Perth 6000
Phone: (08) 9224-2701
Fax: (08) 9224 8009
Epidemiology is so beautiful and provides such an important perspective
on human life and death, 
but an incredible amount of rubbish is published.  Richard Peto (2007) 

-----Original Message-----
[] On Behalf Of Polis,
Chelsea B.
Sent: Thursday, 7 May 2009 7:29 AM
Subject: st: Why don't my IRs and Cox HRs echo each other?

Dear statalisters,

I am doing survival analysis on time to death with time-varying
covariates on an open, population
-based cohort study.  The base sample is essentially a census of
individuals in 56 villages, and
I am utilizing information from all female incident HIV seroconverters.

I computed incidence rates based on the raw data ((number of
deaths/person-time at risk)*100 - I obtained
time at risk using the stdes command), but the IRs don't seem to echo
trends in the univariate Cox HRs.

In the sample data below, things appear reasonable for HC use (deaths
per 100 person years is lower
if HC=yes, and the HR reflects this).  But for current age, deaths are
higher in the 25-34 category
than in the 15-24 category, but the HR trends suggests that being 25-34
is protective (though not
significantly).  Also, the magnitude seems off, for example, in the
variable "Sex partners in past
year" - having two or more seems to more than triple the hazard in the
Cox regression, but merely
increases from 7.46 to 9.10 in the deaths per 100 p-y.

Am I missing something in expecting these numbers to echo trends in each
other?  Is this just a
matter of non-significance within individual categories?  Or a
difference in time-to-event versus
person-time analysis?  Or because I am doing an analysis with
time-varying covariates?  Should I not
expect these to align? Any help is appreciated!

Variable        Deaths  PY at risk      Deaths per      HR      95% CI
                                                100 p-y
HC use
 No             91              1262.7  7.21            1.00
 Yes            13              293.0           4.44            0.58
Current age
 15-24  20              394.0           5.08            1.00
 25-34  49              711.8           6.88            0.73
 35+            35              449.9           7.78            0.68
Sex partners in past year
 None           18              241.2           7.46            1.00
 One            76              1204.6  6.31            1.31
 Two+           10              109.9           9.10            3.40

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