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Selective-Referral and Unobserved Patient Heterogeneity – Bias in the Volume-Outcome Relationship

This paper examines the causal effect of the experience of a hospital with treating hip fractures (volume) on treatment outcome for patients. A full sample of administrative data from Germany for the year 2007 is used. We apply an instrumental variable approach to eliminate endogeneity concerns due to reverse causality and unobserved patient heterogeneity. As instruments for case volume we use the number of potential patients and the number of further hospitals in the region around every hospital. Our results indicate that after application of an IV regression of volume on outcome, volume significantly increases quality.