Uses of ecologic studies in the assessment of intended treatment effects

Because of the potential for confounding by indication (disease severity) i n individual-level observational studies of intended treatment effects, a t reatment designed to prevent an adverse event may appear to cause it. We us e a hypothetical example to show that despite substantial variation in the frequency of treatment among patients residing in different geographic area s, a constant area-specific mortality rate can be observed, indicating the absence of confounding by indication at the ecologic level. The advantage o f ecologic over individual-level observational studies in the assessment of intended treatment effects holds even if variations in disease severity, s ocioeconomic status, and other unmeasured factors are taken into account, a s long as treatment utilization is influenced by practice style in the loca l medical community independently of disease severity. Ecologic studies can suggest the need for changes in practice, help resolve ethical issues, and indicate priorities for randomized trials. J CLIN EPIDEMIOL 52;1;7-12, 199 9. (C) 1999 Elsevier Science Inc.