Assessing socioeconomic effects on different sized populations: To weight or not to weight?

Objective-Researchers in health care often use ecological data from populat ion aggregates of different sizes. This paper deals with a fundamental meth odological issue relating to the use of such data. This study investigates the question of whether, in doing analyses involving different areas, the e stimating equations should be weighted by the populations of those areas. I t is argued that the correct answer to that question turns on some deep epi stemological issues that have been little considered in the public health l iterature. Design-To illustrate the issue, an example is presented that estimates enti tlements to primary physician visits in Manitoba, Canada based on age/gende r and socioeconomic status using both population weighted and unweighted re gression analyses. Setting and subjects-The entire population of the province furnish the data . Primary care visits to physicians based on administrative data, demograph ics and a measure of socioeconomic status (SERI), based on census data, con stitute the measures. Results-Significant differences between weighted and unweighted analyses ar e shown to emerge, with the weighted analyses biasing entitlements towards the more populous and advantaged population. Conclusions-The authors endorse the position that, in certain problems, dat a analyses involving population aggregates unweighted by population size ar e more appropriate and normatively justifiable than are analyses weighted b y population. In particular, when the aggregated units make sense, theoreti cally, as units, it is more appropriate to carry out the analyses without w eighting by the size of the units. Unweighted analyses yield more valid est imations.