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.