

   EExxttrraacctt GGrroouuppiinngg FFaaccttoorrss ffrroomm aann OObbjjeecctt

        getGroups(object, form, level, data)

   AArrgguummeennttss::

     object: any object

       form: an optional formula with a conditioning expression
             on its right hand side (i.e. an expression involv-
             ing the `|' operator). Defaults to `for-
             mula(object)'.

      level: a positive integer vector with the level(s) of
             grouping to be used when multiple nested levels of
             grouping are present. This argument is optional
             for most methods of this generic function and
             defaults to all levels of nesting.

       data: a data frame in which to interpret the variables
             named in `form'. Optional for most methods.

   DDeessccrriippttiioonn::

        This function is generic; method functions can be writ-
        ten to handle specific classes of objects. Classes
        which already have methods for this function include
        `corStruct', `data.frame', `gls', `lme', `lmList', and
        `varFunc'.

   VVaalluuee::

        will depend on the method function used; see the appro-
        priate documentation.

   AAuutthhoorr((ss))::

        Jose Pinheiro and Douglas Bates

   SSeeee AAllssoo::

        `getGroupsFormula'

   EExxaammpplleess::

        ## see the method function documentation

