[Differing opinions about the specification of econometric relationships often lead to a situation in which there are competing non-nested models. This paper is concerned with the problem of testing such models. It is first assumed that tests are based upon instrumental variable estimates (so that the models can be alternative versions of an equation in a system). The tests so derived are then specialized to the case in which ordinary least squares is an appropriate estimator.]

[It is shown that wquilibria with dispersed prices exist in environments with identical and rational agents on both sides of the market. In particular, the original Stigler model of nonsequential search often has many equilibria, some with price dispersion. Also, price dispersion holds in equilibrium in general if search is "noisy," i.e., there is some chance of learning two or more prices when an agent is looking for one price.]