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) 

REQUIRED ARGUMENTS:

...
any number of variables. All must be the same length.

OPTIONAL ARGUMENTS:

na.group
a logical variable, if TRUE, then missing values are treated as a distinct level of each variable.

VALUE:

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

DETAILS:

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

SEE ALSO:

.

EXAMPLES:

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