This function is generic; method functions can be written to handle
specific classes of objects. Classes which already have methods for
this function include:
corStruct,
pdMat and
reStruct
.
USAGE:
getStrataFormula(object, ...)
REQUIRED ARGUMENTS:
object
any object from which a formula representing a
stratification variable can be extracted.
...
some methods for this generic function may require additional
arguments.
VALUE:
a one-sided formula with the stratification variable associated with
object
. For most methods, if no stratification
variable is present, a
NULL value is returned.