The traditional economic approach to index number theory is based on a ratio of cost functions. Diewert defined superlative price and quantity indices as observable indices that were exact for a ratio of unit cost functions or for a ratio of linearly homogeneous utility functions. The present paper looks for counterparts to his results in the difference context, for both flexible homothetic and flexible nonhomothetic preferences. The Bennet indicators of price and quantity change turn out to be superlative for the nonhomothetic case. The underlying preferences are of the translation-homothetic form discussed by Balk, Chambers, Dickenson, Färe, and Grosskopf.
