Estimating the Gravity Model When Zero Trade Flows are Frequent
In this paper we estimate the gravity model allowing for the pervasive issues of heteroscedasticity and zero bilateral trade flows identified in an influential recent paper by Santos Silva and Tenreyro. We use Monte Carlo simulations with data generated using a heteroscedastic, limited-dependent-variable process to investigate the extent to which different estimators can deal with the resulting parameter biases. While the Poisson Pseudo-Maximum Likelihood estimator recommended by Santos Silva and Tenreyro solves the heteroscedasticity-bias problem when this is the only problem, it appears to yield severely biased estimates when zero trade values are frequent. Standard threshold-Tobit estimators perform better as long as the heteroscedasticity problem is satisfactorily dealt with. The Heckman Maximum Likelihood estimators appear to perform well if true identifying restrictions are available.
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