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Improving Observational Study Estimates of Treatment Effects Using Joint Modeling of Selection Effects and Outcomes: The Case of AAA Repair

O'Malley, A. James PhD*; Cotterill, Philip PhD; Schermerhorn, Marc L. MD; Landon, Bruce E. MD, MBA§

doi: 10.1097/MLR.0b013e3182363d64
Original Articles

Background: When 2 treatment approaches are available, there are likely to be unmeasured confounders that influence choice of procedure, which complicates estimation of the causal effect of treatment on outcomes using observational data.

Objective: To estimate the effect of endovascular (endo) versus open surgical (open) repair, including possible modification by institutional volume, on survival after treatment for abdominal aortic aneurysm, accounting for observed and unobserved confounding variables.

Research Design: Observational study of data from the Medicare program using a joint model of treatment selection and survival given treatment to estimate the effects of type of surgery and institutional volume on survival.

Patients: We studied 61,414 eligible repairs of intact abdominal aortic aneurysms during 2001 to 2004.

Measures: The outcome, perioperative death, is defined as in-hospital death or death within 30 days of operation. The key predictors are use of endo, transformed endo and open volume, and endo-volume interactions.

Results: There is strong evidence of nonrandom selection of treatment with potential confounding variables including institutional volume and procedure date, variables not typically adjusted for in clinical trials. The best fitting model included heterogeneous transformations of endo volume for endo cases and open volume for open cases as predictors. Consistent with our hypothesis, accounting for unmeasured selection reduced the mortality benefit of endo.

Conclusions: The effect of endo versus open surgery varies nonlinearly with endo and open volume. Accounting for institutional experience and unmeasured selection enables better decision-making by physicians making treatment referrals, investigators evaluating treatments, and policy makers.

Supplemental Digital Content is available in the text.

*Department of Health Care Policy, Harvard Medical School, Boston, MA

Centers for Medicare and Medicaid Services, Office of Research, Development, and Information, Baltimore, MD

Department of Surgery, Beth Israel Deaconess Medical Center, Boston, MA

§Department of Health Care Policy, Harvard Medical School and Department of Medicine, Beth Israel Deaconess Medical Center, Boston, MA

This study was supported by NIH Grants 1RC4MH092717-01 and 1R01-HL105453 for comparative effectiveness research.

The opinions expressed do not necessarily represent the views or policy positions of the Centers for Medicare and Medicaid Services.

The authors declare no conflict of interest.

Reprints: A. James O'Malley, PhD, Department of Health Care Policy, Harvard Medical School, 180 Longwood Avenue, Boston, MA 02115-5899. E-mail:

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© 2011 Lippincott Williams & Wilkins, Inc.