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
nffGroupedData class is a further refinement that specifies
that the primary covariate is a factor. 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".