R help - Apr 2011 - time dependent hazard ratios

Hi, I am new to time-dependent Cox model to estimate time dependent hazard 
ratios. Let me use aml dataset from survival package:
>
aml3<-survSplit(aml2,cut=c(5,10,20),end="time",start="start",
     event="status",episode="i")

If I want to esimate hazard ratio for each of the time intervals 0-5, 5-10, 
10-20 and >=20, would the following calculate such HRs?
> coxph(Surv(start,time,status)~x*as.factor(i),data=aml3)
Call:
coxph(formula = Surv(start, time, status) ~ x * as.factor(i), 
    data = aml3)


                              coef exp(coef) se(coef)        z  p
xNonmaintained                19.2  2.10e+08    10461  0.00183  1
as.factor(i)1                   NA        NA        0       NA NA
as.factor(i)2                   NA        NA        0       NA NA
as.factor(i)3                   NA        NA        0       NA NA
xNonmaintained:as.factor(i)1 -18.3  1.17e-08    10461 -0.00175  1
xNonmaintained:as.factor(i)2 -19.6  3.18e-09    10461 -0.00187  1
xNonmaintained:as.factor(i)3 -18.0  1.46e-08    10461 -0.00172  1

So my understanding is that:
for time interval 0-5, HR=exp(19.2)=2.1e+08
for time interval 5-10, HR=exp(19.2-18.3)=2.46
for time interval 10-20, HR=exp(19.2-19.6)=0.67
for time interval >=20, HR=exp(19.2-18.0)=3.32

Am I correct on estimating these time dependent HRs?

Thanks

John

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Sorry this is a re-post. I posted it last night, haven't heard from anyone,
hope
this moves the thread up a little and anyone can comment?

Thanks!

John



----- Forwarded Message ----
From: array chip <arrayprofile@yahoo.com>
To: r-help@r-project.org
Sent: Sun, April 17, 2011 11:33:32 PM
Subject: time dependent hazard ratios


Hi, I am new to time-dependent Cox model to estimate time dependent hazard 
ratios. Let me use aml dataset from survival package as an example:
>
aml3<-survSplit(aml,cut=c(5,10,20),end="time",start="start",
     event="status",episode="i")

This will generate the new data frame based on the time cut points 5, 10 and 20 
and produce a indicator variable "i" that indicates the time interval
membership
for each observation.

If I want to esimate hazard ratio of variable "x" (Nonmaintained vs
Maintained)
for each of the time intervals 0-5, 5-10, 10-20 and >=20, would the following
calculate such HRs?
> coxph(Surv(start,time,status)~x*as.factor(i),data=aml3)
Call:
coxph(formula = Surv(start, time, status) ~ x * as.factor(i), 
    data = aml3)


                              coef exp(coef)  se(coef)        z  p
xNonmaintained                19.2  2.10e+08    10461  0.00183  1
as.factor(i)1                   NA        NA        0       NA NA
as.factor(i)2                   NA        NA        0       NA NA
as.factor(i)3                   NA        NA         0       NA NA
xNonmaintained:as.factor(i)1 -18.3  1.17e-08    10461 -0.00175  1
xNonmaintained:as.factor(i)2 -19.6  3.18e-09    10461 -0.00187  1
xNonmaintained:as.factor(i)3 -18.0  1.46e-08    10461 -0.00172  1

So my understanding is that:
for time interval 0-5, HR=exp(19.2)=2.1e+08
for time interval 5-10, HR=exp(19.2-18.3)=2.46
for time interval 10-20, HR=exp(19.2-19.6)=0.67
for time interval >=20, HR=exp(19.2-18.0)=3.32

Am I correct on estimating these time dependent HRs?

Thanks

John

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