Detecting interaction between random region and fixed age effects in disease mapping

The purpose of this article is to draw attention to the possible need for i nclusion of interaction effects between regions and age groups in mapping s tudies. We propose a simple model for including such an interaction in orde r to develop a test for its significance. The assumption of an absence of s uch interaction effects is a helpful simplifying one. The measure of relati ve risk related to a particular region becomes easily and neatly summarized . Indeed, such a test seems warranted because it is anticipated that the si mple model, which ignores such interaction, as is in common use, may at tim es be adequate. The test proposed is a score test and hence only requires f itting the simpler model. We illustrate our approaches using mortality data from British Columbia, Canada, over the 5-year period 1985-1989. For this data, the interaction effect between age groups and regions is quite large and significant.