[R] Centring variables in Cox Proportional Hazards Model

[Laura Bonnett]

Dear All,

I am contemplating centering the covariates in my Cox model to reduce
multicollinearity between the predictors and the interaction term and
to render a more meaningful interpretation of the regression
coefficient.  Suppose I have two indicator variables, x1 and x2 which
represent age categories (x1 is patients less than 16 while x2 is for
patients older than 65).  If I use the following Cox model, is there
anyway I can centre the variables?  Do I have to do it before I fit
them into the model and if so, how?

fit2=coxph(Surv(rem.Remtime,rem.Rcens)~x1(partial)+x2(partial),data=partial,method="breslow")

Thank you,

Laura