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Analysis Using Instrumental Variables

Mark McClellan, MD, PhD; Barbara J. McNeil, MD, PhD; Joseph P. Newhouse, PhD

JAMA. 1994;272(11):859-866.

Abstract
Objective.
—To determine the effect of more intensive treatments on mortality in elderly patients with acute myocardial infarction (AMI).

Design.
—Analysis of incremental treatment effects using differential distances as instrumental variables to account for unobserved case-mix variation (selection bias) in observational Medicare claims data (1987 through 1991).

Main Outcome Measures.
—Survival to 4 years after AMI.

Results.
—Patients who receive different treatments differ in observable and unobservable health characteristics, biasing estimates of treatment effects based on standard methods of adjusting for observable differences. Patients' differential distances to alternative types of hospitals are strong independent predictors of how intensively an AMI patient will be treated and appear uncorrelated with health status. Thus, differential distances approximately randomize patients to different likelihoods of receiving intensive treatments. Comparisons of patient groups that differ only in differential distances show that the impact on mortality at 1 to 4 years after AMI of the incremental ("marginal") use of invasive procedures in Medicare patients was at most 5 percentage points; this gain was achieved during the first day of hospitalization and therefore appears attributable to treatments other than the procedures. Admission to a hospital treating a high volume of AMI patients was associated with an effect on mortality at 4 years of less than 1 percentage point, again arising on day 1. Patients living in rural areas experienced acute mortality that was an additional 0.6 percentage-point higher, after controlling for less access to intensive treatments.

Conclusions.
—For elderly patients with AMI, the aspects of treatment most affecting long-term survival relate to care within the first 24 hours of admission. The survival benefits from greater use of catheterization and revascularization procedures appear minimal in marginal patients.

(JAMA. 1994;272:859-866)

Author Affiliations
From the Department of Health Care Policy, Harvard Medical School (Drs McClellan, McNeil, and Newhouse), the Department of Health Policy and Management, Harvard School of Public Health (Dr Newhouse). the Departments of Medicine (Dr McClellan) and Radiology (Dr McNeil), Brigham and Women's Hospital, Boston, Mass; the John F. Kennedy School of Government, Harvard University (Dr Newhouse), and the National Bureau of Economic Research (Dr McClellan), Cambridge, Mass.

Footnotes
Address correspondence to National Bureau of Economic Research, 1050 Massachusetts Ave, Cambridge, MA 02138 (Dr McClellan).

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