Distortion of regional old-age mortality rates due to migration in the period prior to death in the Netherlands?
BACKGROUND Regional mortality differences are an important public health indicator and can serve as the basis for population forecasts and local planning. Health-related migration at old age may distort observed regional mortality. OBJECTIVE We assess whether or not internal migration in late life has the potential for distortion of regional old-age mortality rates and differences therein. DATA AND METHODS Using data from the Dutch population register, we analyzed migration and death rates in the population aged 80 years and above across the Dutch NUTS 2 regions, NUTS 3 regions and municipalities in the years 2002 through 2006. Observed sex-specific age-standardized death rates-and regional differences therein-are compared to hypothetical mortality rates, which disregard internal migration in the three years prior to death. RESULTS Late-life internal migration intensities were highest at municipality level, for women, and for those who died. Almost half of the municipalities showed differences of more than +/-5% between the observed and hypothetical mortality rates. Many of the municipalities, whose observed mortality rates differed significantly from the Dutch average, displayed no significant difference from the average in the hypothetical scenario. Regional mortality variation across the municipalities decreased significantly if late-life migration was disregarded. Differences were larger for women than men. Minimal differences appeared between the observed situation and hypothetical scenario at NUTS 2 and NUTS 3 levels. CONCLUSION Late-life migration can significantly distort regional old-age mortality. Mortality risk factors therefore appear to be more equally distributed than the observed rates imply. The bias depends on age, regional level, migration intensity, and the role of nursing care.