strata {survival5}R Documentation

Identify Strata 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=F)

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.

Details

The result is identical to the interaction function, but for the labeling of the factors (strata is more verbose).

Value

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

Examples

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

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