THE SEMI-INDIVIDUAL STUDY IN AIR-POLLUTION EPIDEMIOLOGY - A VALID DESIGN AS COMPARED TO ECOLOGIC STUDIES

The assessment of long-term effects of air pollution in humans relies on epidemiologic studies. A widely used design consists of cross-secti onal or cohort studies in which ecologic assignment of exposure, based on a fixed-site ambient monitor, is employed. Although health outcome and usually a large number of covariates are measured in individuals, these studies are often called ecological. We will introduce the term semi-individual design for these studies. We review the major propert ies and limitations with regard to causal inference of truly ecologic studies, in which outcome, exposure, and covariates are available on a n aggregate level only. Misclassification problems and issues related to confounding and model specification in truly ecologic studies limit etiologic inference to individuals. In contrast, the semi-individual study shares its methodological and inferential properties with typica l individual-level study designs. The major caveat relates to the case where too fem study areas, e.g., two or three, are used, which render control of aggregate level confounding impossible. The issue of expos ure misclassification is of general concern in epidemiology and not an exclusive problem of the semi-individual design. In a multicenter set ting, the semi-individual study is a valuable tool to approach long-te rm effects of air pollution. Knowledge about the error structure of th e ecologically assigned exposure allows consideration of the impact of ecologically assigned exposure on effect estimation. Semi-individual studies, i.e., individual level air pollution studies with ecologic ex posure assignment, more readily permit valid inference to individuals and should not be labeled as ecologic studies.