Extract varFunc Groups

DESCRIPTION:

This method function extracts the grouping factor associated with the variance function represented by object, if any is present.

USAGE:

getGroups(object) 

REQUIRED ARGUMENTS:

object
an object inheriting from class varFunc, representing a variance function structure.

VALUE:

if object has a groups attribute, its value is returned; else NULL is returned.

EXAMPLES:

vf1 <- varPower(form = ~ age | Sex) 
vf1 <- initialize(vf1, Orthodont) 
getGroups(vf1)