If present, the grouping factor associated to the correlation
structure for the linear model represented by
object is extracted.
USAGE:
getGroups(object)
REQUIRED ARGUMENTS:
object
an object inheriting from class
gls, representing
a generalized least squares fitted linear model.
VALUE:
if the linear model represented by
object incorporates a
correlation structure and the corresponding
corStruct object
has a grouping factor, a vector with the group values is returned;
else,
NULL is returned.