Calculating marginal effects in models for zero expenditures in household budgets using a Heckman-type correction

Using the Heckman approach, either in single-equation or multi-equation settings, general expressions are derived for calculating marginal effects and elasticities. In the conventional calculation of marginal effects, terms related to the change in the inverse of Mills ratio are omitted. Using data from the 1987-88 Nationwide Food Consumption Survey, we calculate income and household size elasticities for 12 food commodities. We compare the magnitudes and signs of the elasticities using the conventional expressions of marginal effects and our derived expressions. Bottomline, sizeable differences, especially in single-equation applications, can occur in calculating marginal effects if one fails to account for changes in the inverse of the Mills ratio.

Overview