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


From   "Carlo Lazzaro" <carlo.lazzaro@tiscalinet.it>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: R: RE: Why don't my IRs and Cox HRs echo each other?
Date   Thu, 7 May 2009 07:52:33 +0200

Dear Chelsea,
folowing Kieran's helpful advice, besides Kaplan-Meier estimates, you could
take a look at whether deaths in one out of the compared groups tended to
occur earlier or later vs the other one via log-rank test (more sensitive to
differences occurring at the end of the follow-up) and Wilcoxon test (more
sensitive to differences occurring at the beginning of the follow-up).

Amongst other contributions, this topic is well covered in Maarten L. Buis.
An introduction to Survival Analysis. 2006 (http://home.fsw.vu.nl/m.buis/)
Cleves MA, Gould WG, Gutierrez R. An Introduction To Survival Analysis Using
Stata. Revised edition. College Station: StataPress, 2006.

HTH and Kind Regards,
Carlo
-----Messaggio originale-----
Da: owner-statalist@hsphsun2.harvard.edu
[mailto:owner-statalist@hsphsun2.harvard.edu] Per conto di Polis, Chelsea B.
Inviato: giovedì 7 maggio 2009 1.35
A: statalist@hsphsun2.harvard.edu
Oggetto: st: RE: Why don't my IRs and Cox HRs echo each other?

I apologize for the formatting of my table, it looked ok when I sent it.
Please let me try again.


Variable  Deaths  PY at risk    IR      HR      95% CI  p-value

HC use                                                          0.07
 No         91  1262.7  7.21    1.00
 Yes        13  293.0           4.44    0.58    0.32-1.04
Current age                                                             0.38
 15-24    20    394.0           5.08    1.00
 25-34    49    711.8           6.88    0.73    0.43-1.24
 35+        35  449.9           7.78    0.68    0.38-1.20
Sex partners in past year                                       0.01
 None       18  241.2           7.46    1.00
 One        76  1204.6  6.31    1.31    0.78-2.21
 Two+       10  109.9           9.10    3.40    1.54-7.54

-----Original Message-----
From: owner-statalist@hsphsun2.harvard.edu
[mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Polis, Chelsea B.
Sent: Wednesday, May 06, 2009 7:29 PM
To: statalist@hsphsun2.harvard.edu
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
p-value
                                                100 p-y
HC use
0.07
 No             91              1262.7  7.21            1.00
 Yes            13              293.0           4.44            0.58
0.32-1.04
Current age
0.38
 15-24  20              394.0           5.08            1.00
 25-34  49              711.8           6.88            0.73    0.43-1.24
 35+            35              449.9           7.78            0.68
0.38-1.20
Sex partners in past year
0.01
 None           18              241.2           7.46            1.00
 One            76              1204.6  6.31            1.31    0.78-2.21
 Two+           10              109.9           9.10            3.40
1.54-7.54


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