Level of aggregation for optimal epidemiological analysis: the case of time to surgery and unnecessary removal of the normal appendix

摘要

Study objective-To illustrate the concept of "individualised fallacy", the result of improper interpretation and inference about aggregate level associations on the basis of associations at the individual level, in epidemiology. Design-Cohort study. Setting-Canadian province of Ontario. Patients-All patients who underwent pri- Mary appendicetomy in 175 Ontario hos- Pitals from 1989 to 1992. The association Between rate of normal appendix removal And time to surgery was analysed at two Levels: (1) at individual patient level, in Which, for each patient, the exact number Of days to surgery was derived, and (2) at Hospital level, in which hospital specific Proportions of time to surgery was calcu- Lated.