Health Services and Outcomes Research Methodology

, Volume 2, Issue 3, pp 221–245

Causal Effect of Ambulatory Specialty Care on Mortality Following Myocardial Infarction: A Comparison of Propensity Score and Instrumental Variable Analyses

  • Mary Beth Landrum
  • John Z. Ayanian

DOI: 10.1023/A:1020367111374

Cite this article as:
Landrum, M.B. & Ayanian, J.Z. Health Services & Outcomes Research Methodology (2001) 2: 221. doi:10.1023/A:1020367111374


The quality and outcomes of care provided by primary care physicians and specialists are increasingly important issues in health policy research. Estimating the effect of specialty care on patient outcomes however is complicated by the observational nature of the studies. Patients treated by specialists are often different in terms of observed and unobserved characteristics that can bias estimates of specialty effects. We illustrate and compare two different analytic approaches, propensity scores and instrumental variables, to infer the causal effect of cardiology care in the ambulatory setting on 18-month mortality among 5467 elderly patients who survived at least 3 months after being hospitalized for a myocardial infarction in New York state during 1994 and 1995. Using both approaches we found reductions in 18-month mortality associated with ambulatory cardiology care. However, reasonable deviations from the assumptions underlying each method led to estimated differences in mortality ranging from a 6% absolute reduction in mortality to a 2% increase among patients who received cardiology care. Choosing an analytic strategy depends on both available data and the policy question of interest. We believe that comparative analyses such as this one, with extensive assessment of the assumptions underlying each method, can provide valuable insights into important policy questions reliant on the analysis of observational data.

causal inferencepropensity scoresinstrumental variablesacute myocardial infarctionspecialty care

Copyright information

© Kluwer Academic Publishers 2001

Authors and Affiliations

  • Mary Beth Landrum
    • 1
  • John Z. Ayanian
    • 1
    • 2
  1. 1.Department of Health Care PolicyHarvard Medical SchoolBostonUSA
  2. 2.Department of Medicine, Division of General MedicineBrigham and Women's Hospital and Harvard Medical SchoolUSA