There is a discussion in Appendix D.3 of "Modeling Survival Data" by
Thereau and Grambsch regarding the differences in the datasets
including the fact that "there was significantly more follow-up for
many patients at the time this dataset was assembled". I do not see a
material difference in the estimates.
--
David Winsemius, MD
Heritage Labs
On Nov 21, 2008, at 12:16 PM, Ravi Varadhan wrote:
Hi,
When I run the following Cox proportional hazards model on the Mayo
clinic's
PBC data set (given in the "survival" package), the regression
coefficients
do not agree with the results presented in Table 4.6.3 (p. 195) of
Fleming &
Harrington's book.
library(survival)
data(pbc)
ans.cox <- coxph(Surv(time, status) ~ log(bili) + log(alb) + age +
log(protime) + edema)
ans.cox
ans.cox <- coxph(Surv(time, status) ~ log(bili) + log(alb) + age +
log(protime) + edema)
ans.cox
Call:
coxph(formula = Surv(time, status) ~ log(bili) + log(alb) + age +
log(protime) + edema)
coef exp(coef) se(coef) z p
log(bili) 0.8975 2.453 0.08271 10.85 0.0e+00
log(alb) -2.4524 0.086 0.65707 -3.73 1.9e-04
age 0.0382 1.039 0.00768 4.97 6.5e-07
log(protime) 2.3458 10.442 0.77425 3.03 2.4e-03
edema 0.6613 1.937 0.20595 3.21 1.3e-03
Likelihood ratio test=234 on 5 df, p=0 n= 418
These coefficients, however, are significantly different (i.e. the
differences can't be just attributed to round-off's) from that
reported in
Table 4.6.3 (in the "Final model" column) of Fleming and Harrington
(p.
195). The coefficients reported are: 0.8707, -2.533, 0.0394, 2.380,
0.8592.
Note the big difference for the "edema" variable.
It seems like the data set considered in the book and that available
in
"survival" package are the same (with n=418).
I also re-ran the Cox PH model with the 2 "data-errors" discussed in
p.188
of F&H, but still I could not match the results in Table 4.6.3.
Is it possible that the results could be explained due to difference
in
convergence during maximization of partial likelihood?
Can anyone help me figure out why this diescrepancy exists?
Thanks very much,
Ravi.
----------------------------------------------------------------------------
-------
Ravi Varadhan, Ph.D.
Assistant Professor, The Center on Aging and Health
Division of Geriatric Medicine and Gerontology
Johns Hopkins University
Ph: (410) 502-2619
Fax: (410) 614-9625
Email: [EMAIL PROTECTED]
Webpage: http://www.jhsph.edu/agingandhealth/People/Faculty/Varadhan.html
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