This paper derives the marginal effects for a conditional mean function in a bivariate probit model. A general expression is given for a model which allows for sample selectiviy and heteroscedasticity. The computations are illustrated using microeconomic data from a study on credit scoring.

We analyze the equilibrium size of networks under alternative market structures. Networks are characterized by positive size externalities (commonly called "network externalities"). That is, the benefits of the addition of an extra node (or an extra customer) exceed the private benefits accruing...