Sample Selection in the Poisson Regression Model

We present a correction for sample selectively in the poisson regression model for count data. The model is similar to that devised by Heckman for the linear regression model. Estimation by a two step method is suggested using nonlinear least squares at the second step. The model described here was presented in Greene(1994). Terza(1995) describes an alternative approach that has more orthodox specification of the regression function. We show in this note that Terza's approach is essentially the same as Greene's