Variable names in model formula

At 10:37 PM 5/1/00 -0400, E. S. Venkatraman wrote:

I have the following problem.  I have survival data (time, status) along
with several covariates (X1, X2,..., Xn).  I want to fit a Cox model for
each of the covariate (univariately) and obtain the fitted probability of
survival at a fixed time point t0 and covariate value Xi0.  I tried to do
this in a for loop where the index is the variable name as follows

covnames <- names(...)
for (covi in covnames) {
 survout <- coxph(Surv(time,status) ~ covi, data = dataname)
 print(summary(survfit(survout,newdata),t0))
}

You could try

covnames <- names(...)
form <- Surv(time, status) ~ dummy
for(nam in covnames) {
	form[[3]] <- as.name(nam)
	survout <- coxph(form, data = dataname)
	print(summary(survfit(survout, newdata), t0))
}

A related idea, which doesn't require you to know the structure of formula
object, but will be slower, is

covnames <- names(...)
for(nam in covnames) {
	form<-substitute(Surv(time,status)~dummy,list(dummy=nam))
	survout<-coxph(form,data=dataname)
	print(summary(survfit(survout, newdata), t0))
}

This method also works if you want to loop over different time and status
variables.

	-thomas

Thomas Lumley
Assistant Professor, Biostatistics
University of Washington, Seattle

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