untangle.specials {survival}  R Documentation 
Help Process the 'specials' Argument of the 'terms' Function.
Description
Given a terms
structure and a desired special name, this returns an
index appropriate for subscripting the terms
structure and another
appropriate for the data frame.
Usage
untangle.specials(tt, special, order=1)
Arguments
tt 
a terms object.

special 
the name of a special function, presumably used in the terms object.

order 
the order of the desired terms. If set to 2, interactions with the special
function will be included.

Value
a list with two components:
vars 
a vector of variable names, as would be found in the data frame, of the
specials.

terms 
a numeric vector, suitable for subscripting the terms structure, that indexes
the terms in the expanded model formula which involve the special.

Examples
formula<Surv(tt,ss)~x+z*strata(id)
tms<terms(formula,specials="strata")
## the specials attribute
attr(tms,"specials")
## main effects
untangle.specials(tms,"strata")
## and interactions
untangle.specials(tms,"strata",order=1:2)
[Package
survival version 2.17
Index]