Count data models, such as the Poisson and negative binomial, have been suggested for estimating the number of occurrences of an event, or event counts (see e.g., Wooldridge, 2002; Cameron and Trivedi, 1998; Hausman, Hall and Griliches, 1984; Maddala, 1983:51). Formally, the Poisson model is derived by assuming that a random variable y is Poisson-distributed with the conditional density of y equal to (y|x) = (e~°0^ )y!, The log of the mean 0 is assumed to be a linear function of a vector of independent regressors x: In 0 = x’/3, where 0 is a parameter vector.