The Log Normal and the Poisson Gravity Models in the Analysis of Interactions Phenomena

Three problems often encountered when bilateral interaction data are analyzed by means of the log-normal gravity model: the bias created by the logarithmic transformation, the failure of the homoscedasticity assumption and the treatment of zero valued flows. When the interaction are count data type that takes non-negative integer values, to overcome these problems the literature suggests to use a Poisson gravity model instead of log-normal model. In this paper, using a real interaction phenomenon a comparative analysis of the two models is carried out. The most important results obtained highlights that if the phenomenon is correctly specified, the two specification of the gravity model have a very similar behaviour.