A simple regression test of the null hypothesis of no sample selection bias has been suggested by J. J. Heckman. Because of its computational simplicity, this test is widely used by applied researchers. This paper shows that Heckman's test is equivalent to the Lagrange multiplier test of the null hypothesis. This allows us to readily deduce that the simple regression test also has desirable large sample properties.

DESPITE THE INTENSE ACTIVITY in the area of estimation of limited dependent variable (LDV) models (see, for example, the Fall 1976 issue of the Annals of Economic and Social Measurement), the interpretation of regression coefficients in truncated regression models has been largely ignored. This issue should not be taken lightly since the obvious interpretation which equates regression coefficients to partial derivatives of the conditional mean of the dependent variable is unfortunately incorrect. One specific implication of the results presented here is that whenever the dependent variable y in the usual classical linear normal regression model is truncated above and/or below, then the effect of the fth regressor on the conditional mean of y is proportional to, but not equal to, the fth regression coefficient. However, more generally this note presents a simple expression for the effect of the fth regressor on any conditional moment of y in terms of a generalized LDV model introduced by Poirier [3]. Poirier introduced a general LDV model which permitted skewness in a pre-truncated variable by transforming it within the class of transformations suggested by Box and Cox

Hazard-function estimates are utilized to analyze the effect of numerous public policy variables on strike duration, based on 7,546 strikes in Canada between 1967 and 1985. We find that only the mandatory strike vote substantially reduced conditional duration. However, the policy variables generally had a more favorable effect on reducing strike incidence, so that on net they tended to reduce unconditional duration (incidence times conditional duration). Specifically, reduced unconditional duration was associated with the existence of conciliation and the requirement of a mandatory strike vote, but the prohibition on replacement workers had a perverse effect.

Estimates of the effect of strikes on the production, price, and purchases of North American automobiles are provided over the period 1966-79. The estimates are based on a model that reflects the decisions of both consumers and producers and that captures important intertemporal adjustments before, during, and after the strike to allow for inventory buildup in anticipation of the strike as well as inventory depletion during the strike and replacement afterward. The results generally indicated that both consumers and producers rationally responded to the expected and the actual event of the strike through a variety of related intertemporal adjustments; and, while the initial effects are in some instances quite pronounced, the long-run effects are usually minimal.