what is ref.df in r? how to find r ref.df what is ref-df in r what is ref.df in r what is ref df what is ref.df agm what is ref.df in r output what is ref.df what is ref.df in r what is ref.df generalised additive model what does ref mean define ref df in r what is ref.df in r ref.df in r what is ref.df definition of ref.df in r what is ref.df in r what is ref.df output? what is ref.df in r what is ref df