On Clarifying M and M

Second only to learning from one's mistakes is the benefit to be gained from those of one's friends and colleagues. Since Massy and Morrison [ 4] provide no instances of the first and few of the second, their comments on Markov and me [ 1] are less valuable than they might have been. Only a few points remain to be clarified. One such point is the stochastic or deterministic formulation of buyer behavior models. About Markov it is agreed that it would be preferable if a first-order stationary brand-switching model could be formulated stochastically, but it is also agreed that this is not really possible-"his criticisms are technically correct." Some people do however treat Markov determinz'stically, see [3]. Massy and Morrison want to dismiss this deterministic formulation as untenable on a priori grounds and seem worried because I examined it on its merits. They fear that by treating determinism with such respect I might seem to actually like it. Such a misinterpretation seems improbable; but since it is clearly possible, I recapitulate: I would accept a deterministic formulation of the Markovian brand-switching model as a working