groupedData object is at least a data.frame and a
  display  formula that provides a grouping structure.  In the
  nfGroupedData class there is only one level of grouping factors
  and the response in the display formula is numeric.  The
  nfnGroupedData class is a further refinement that specifies
  that the primary covariate is numeric. At present we
  are using old-style classes for groupedData and the classes
  that extend it."nfGroupedData", directly.
Class "groupedData", by class "nfGroupedData".
Class "data.frame", by class "nfGroupedData".
Class "oldClass", by class "nfGroupedData".