survival: strata – R documentation

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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.

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)

survival

Survival Analysis

v3.2-11

LGPL (>= 2)

Authors

Terry M Therneau [aut, cre], Thomas Lumley [ctb, trl] (original S->R port and R maintainer until 2009), Atkinson Elizabeth [ctb], Crowson Cynthia [ctb]

Initial release

2021-04-25