Combining ecological and individual variables to reduce confounding by indication: Case study - subarachnoid hemorrhage treatment

Ecological studies may reduce the problem of confounding by indication. how ever, these studies introduce new biases not present in individual-level an alyses. To study the potential for ecological Variables to reduce confoundi ng by indication, we used a large database of admissions for ruptured cereb ral aneurysms to evaluate the association of in-hospital death with treatme nt type-surgery or endovascular therapy. We compared results of three multi variable models: individual-level, ecological, and a two-level model with a n ecological treatment variable and individual-level covariates and outcome . Trends in the individual-level and ecological models were in opposite dir ections. suggesting confounding by indication in the individual-level analy sis. The two-level analysis revealed a strong association between instituti onal utilization of endovascular therapy and reduced individual risk of in- hospital death. Using an ecological treatment variable in an individual ana lysis may combine reduced confounding by indication in ecological analyses with increased power and more precise specification of outcomes and covaria tes in individual-level analyses. (C) 2000 Elsevier Science Inc. All right reserved.