We examine how the choice of areal unit affects the estimation of neighbourhood effects on mortality using two different areal units. We used register data of 70,936 individuals aged 25-64 years residing in the capital region of Helsinki, Finland. Results from the multilevel Poisson regression show that the clustering of mortality was slightly stronger when using smaller area units. The differences disappear when account was taken of known individual-level characteristics of the residents. This was also the case for the effect of the proportion of manual workers in the area on mortality. Our results imply that the choice of area scale will not lead to serious underestimation of neighbourhood effects in mortality.
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