[R] percent inclusion in cph

[sushi4u]

Dear R-help,

I have to calculate the percent inclusion of each variable in a bootstrap validation of a cox proportional hazards model(described in Sauerbrei and Schumacher, Stat Med 11:1093, 1992).

First I need to get a bootstrap sample from my dataset, which I did with the sample function. Then I tried to run a cph model and looked which covariates are significant. This I would repeat 200 times and at the end calculate how many percent the covariates were included. 

This is what I entered:
boot1 <- sample(Dataset, 300, replace=T)
cph1 <- cph(Surv(months,status) ~ cov1 + cov2 + cov3 + cov4, data=boot1)

Unfortunately, I get exactly the same results (coefficient, SE, p-value) then when I would fit a Cox model without drawing a bootstrap sample before.

How do I do it right? Or is there another way to calculate the percentage?

Sorry for my bad English. 

Thanks.

Dott. Mario Rossi
University of Foggia, Italy