strata Identify Stratification Variables

Description

This is a special function used in the context of the Cox survival model. It identifies stratification variables when they appear on the right hand side of a formula.

Usage

strata(..., na.group=FALSE, shortlabel, sep=', ')

Arguments

...	any number of variables. All must be the same length.
na.group	a logical variable, if TRUE, then missing values are treated as a distinct level of each variable.
shortlabel	if TRUE omit variable names from resulting factor labels. The default action is to omit the names if all of the arguments are factors, and none of them was named.
sep	the character used to separate groups, in the created label

Details

When used outside of a coxph formula the result of the function is essentially identical to the interaction function, though the labels from strata are often more verbose.

Value

a new factor, whose levels are all possible combinations of the factors supplied as arguments.

See Also

coxph, interaction

Examples

a <- factor(rep(1:3,4), labels=c("low", "medium", "high"))
b <- factor(rep(1:4,3))
levels(strata(b))
levels(strata(a,b,shortlabel=TRUE))

coxph(Surv(futime, fustat) ~ age + strata(rx), data=ovarian)