Do differences in the administrative structure of populations confound comparisons of geographical health inequalities?
posted on: Aug 31, 2010
A new study identifies a problem that could distort international comparisons of health inequalities.
Geographic health inequalities are naturally described by the variation in health outcomes, such as mortality, between areas. However, one might rightly question whether it is fair to make direct comparisons of health inequalities between areas which have different internal administrative unit sizes. Our objective was to assess how differences in geographical and administrative units affect the description of health inequalities. Countries with larger between-region variation in mortality rates have greater geographic inequalities in this health measure than countries with lower variation; we hypothesize that this variation will be related to the distribution of internal region sizes since ecological determinants of health will be more strongly clustered in small regions than large ones.
We examined whether spatial inequalities in mortality between 20 European countries were associated with the distribution in regional population sizes. We used multilevel statistical methodology to explicitly model how within-country variation in mortality relates to mean region population size and to a measure of inequality in region population size. Our results suggest that countries or areas comprising unequally sized geographic units may have the tendency to show greater health inequalities simply because of inherent differences in geographic structure. It is important to be aware of the potential for such biases when examining health inequalities between areas; we recommend that when presenting such health inequalities one should report a simple measure of inequities in population structure, such as the Gini coefficient. Additionally we have provided a statistical method which adjusts for this bias by allowing for the variation in administrative unit sizes within larger areas. We expect widespread differences between countries in terms of the geographical units used for reporting data and hence the description of health inequalities may be more affected by the hierarchical structure than considered previously. A better understanding of how differences in geographic structure of a population affect the description and interpretation of health inequalities will improve spatio-temporal monitoring of health inequalities and better inform the evaluation of interventions.
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