[R] Two ways to deal with age in Cox model

[John Sorkin]

I hope one and all will allow a stats question:

When running a cox proportional hazards model ,there are two ways to
deal with age, 
including age as a covariate, or to include age as part of the
follow-up time, viz,

Age as a covariate:

tetest1 <- list(time=  c(4, 3,1,1,2,2,3),
                     status=c(1,NA,1,0,1,1,0),
                     age= c(0, 2,1,1,1,0,0),
                     riskfactor=   c(0, 0,0,0,1,1,1))
 fitagecovariate<-coxph( Surv(time, status) ~ age +riskfactor, test1) 
 fitagecovariate

Age included as part of follow-up time:

 test2<-test1
 test2$timeplusage<-test2$time+test2$age
 fitagefollowup<-coxph( Surv(timeplusage, status) ~ riskfactor, test2)
 fitagefollowup

I would appreciate any thoughts about the differences in the
interpretation of the two models.
One obvious difference is that in the first model (fitagecovariate) one
can make inferences about age and in the second one cannot. I think a
second
difference may be that in the first model the riskfactor is assumed to
have values measured at the values of age where as in the second model
riskfactor is assumed to have given values throughout the subject's
life.

Your thoughts please.

Thanks,
John

R 2.1.1
windows XP

John Sorkin M.D., Ph.D.
Chief, Biostatistics and Informatics
Baltimore VA Medical Center GRECC,
University of Maryland School of Medicine Claude D. Pepper OAIC,
University of Maryland Clinical Nutrition Research Unit, and
Baltimore VA Center Stroke of Excellence

University of Maryland School of Medicine
Division of Gerontology
Baltimore VA Medical Center
10 North Greene Street
GRECC (BT/18/GR)
Baltimore, MD 21201-1524

(Phone) 410-605-7119
(Fax) 410-605-7913 (Please call phone number above prior to faxing)
jsorkin at grecc.umaryland.edu

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