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

Abstract. 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