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The exposure fallacy: migration, mobility and ecological analysis of health status in Ireland

Frank Houghton, Kevin Kelleher


The problem of differential rates of exposure within areas resulting from spatially inconsistent rates of migration and mobility is largely ignored in ecological health analysis. This error may be termed the 'exposure fallacy'. This paper quantifies population mobility and migration in Ireland using a variety of census measures. The results indicate that there is not only substantial residential mobility in Ireland, but that the nature and extent of this mobility differs significantly across areas. For example in 1996 more 250,000 people were not living in the same residence they had occupied twelve months previously. The viability of ecological analysis is therefore questioned. As anticipated the most mobile groups were young adults aged 20-35 years, and most residential moves were of a local nature. The error associated with the exposure fallacy may be partially reduced if analysis is conducted at county level and if young adults are excluded. The development of an area based mobility index for Ireland is also proposed for use in weighted least squares regression analysis. However significant problems remain despite these precautions. The adoption of a Finnish style centralised population and housing register is proposed, as is the introduction of a unique identifier for all individuals within the State.

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