IMPROVING DISTANCE ESTIMATES BETWEEN AREAL UNITS IN MIGRATION MODELS

There are many methods of modeling migrant flows within a set of areal units, but it is common in most to incorporate some measure of distan ce as an explanatory variable. These distances are effectively meant t o represent the typical distance between pairs of areas that would be traveled by potential migrants. They are usually calculated between po pulation-weighted centroids derived for each zone. It is argued here t hat this method of calculating distance is biased and that the zonal s ystem used will influence the Fnal model parameters that are intended to describe the underlying migration process. The distances between ne arby zones will be particularly poorly specified using this approach, but other problems arise which relate to the shape of the zones and th e position of the zones in relation to each other. This paper describe s an alternative method of calculating these distances which reduces t his bias. It is shown that the resulting models fit the data far more satisfactorily and that the residuals from models incorporating this a pproach are significantly different from those identified from models that use the standard method of specifying distance.