Hi everyone,
I have a doubt about the way to calculate 95% CI for coefficients in
the stratified Cox proportional hazard models and your help is welcomed
.
Say that I have a variable of interest Imi and a stratifying variable
UV in a interaction model (since the interaction between Imi and UV is
of interest for me and the interaction model has a better fit than the
no-interaction model:
library(survival)
model1 <- coxph(Surv(start,stop, Status.time)?
???????????????????~ Imi + Imi:UV + strata(UV)?+ cluster(ID),?
? ? ? ? ? ? ? ? ? ? ?weights = NB_Event, data=Data.unfold)?
Whereas it is pretty straightforward to calculate the coefficients (and
associated HR) for each combination of Imi and UV, I am not sure about
how to calculate the associated CI (note that, of course, I got the CI
for the estimate of "Imi:UV" from the output of model1).?
Is it correct to calculate separately a model for each UV level and use
the CI for the Imi variable to get the CI for the two levels of UV (see
below) ? ?
# 2 models (one per UV level)?
data.Low <- subset(Data.unfold,UV=="low")
model2.1 <- coxph(Surv(start,stop, Status.time)?
???????????????????~ Imi + cluster(ID),?
? ? ? ? ? ? ? ? ? ? ?weights =
NB_Event, data=data.Low)?
data.High <- subset(Data.unfold,UV=="high")
model2.2 <-
coxph(Surv(start,stop, Status.time)?
???????????????????~ Imi +
cluster(ID),?
? ? ? ? ? ? ? ? ? ? ?weights = NB_Event, data=data.High)?
Alternatively, is there a way to get the CI directly from the output of the stratified model ?
Many thanks?
Andrea Bertolo
Universit? du Qu?bec ? Trois-Rivi?res
3351, bd des Forges
C.P.500, Trois-Rivi?res (Qu?bec) Canada
G9A 5H7