COMPUTING AND USING RURAL VERSUS URBAN MEASURES IN STATISTICAL APPLICATIONS

We discuss various measures of urban-versus-rural character, particula rly when used for adjustment of epidemiologic or environmental data. F or example, because urban-versus-rural differences may conceal more su btle effects, such a measure may be used to adjust for urban-versus-ru ral differences in county mortality rates, so that underlying geograph ic or demographic patterns may be revealed. Urban-versus-rural measure s frequently have been defined as categorical variables given by range s of (i) total population; (ii) population density; and (iii) percent urban population. When regions are counties in the United States, such measures are easily obtained, but their relation to one's concept of urban-versus-rural character is often weak, as illustrated with severa l examples. A class of alternative measures is proposed. These measure s are functions of the individual populations of places (usually minor civil divisions) in the county: (1) population of the largest place; (2) root sum of squared populations of three largest places; and (3) r oot sum of squared populations of all places. Two members of this clas s already have shown promise in statistical analyses of lung cancer mo rtality rates, namely (1) and (2). Using U.S. county data, we demonstr ate that (3) adds only very marginal information to the measure, and t hat (2), which we call urbanicity, is useful for purposes of adjustmen t in studies of ecological exploration.