It is often difficult to justify a maintained hy­

pothesis that the regression c o e f f i c i e n t s are

stable over a sample interval. Stability(of esti­

mated functional form)of economic relations is

largely an empirical question. Stability is also

closely connected with the e m p i r i c a l issue of

choosing among appropriate definitions of the de­

pendent and independent variables in the model.