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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.
strata(..., na.group=FALSE, shortlabel, sep=', ')a new factor, whose levels are all possible combinations of the factors supplied as arguments.
any number of variables. All must be the same length.
a logical variable, if TRUE, then missing values are treated as a
distinct level of each variable.
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.
the character used to separate groups, in the created label
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.
coxph, interaction
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)
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