MIGRATION MODELS INCORPORATING INTERDEPENDENCE OF MOVERS

Models of migration between regions are often based on the assumption that individual moves can be modelled by a Poisson distribution whose parameter is a function of origin and destination characteristics, and generalized cost; this is true of Poisson regression models and spati al interaction models. The Poisson assumption is that each individual acts independently from others making the same move. In fact, migratio n is usually engaged in by household groups, not independent individua ls, making the Poisson assumption invalid. It is possible instead to c onstruct a model in which the probability of a household moving is giv en by a Poisson model and the number of individuals in a moving househ old is given by an observed house-hold size distribution. This general ized Poisson model is explained and fitted to a set of data on local-l evel migration within the English county of Hereford and Worcester. Ho wever, the sparse nature of the data set raises problems in assessing goodness of fit because the deviance value is unusually low. This is t ackled here with a simulation methodology.