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OBJECTIVE: To examine the effect of graduate medical education sponsorship on hospital operating costs over a seven-year period, to test for a longitudinal association between teaching intensity and cost, and to determine whether the indirect medical education (IME) payment adjustments made under Medicare's Prospective Payment System are appropriate. DATA SOURCES: Medicare cost and payment data from the Hospital Cost Report Information System and other related HCFA files, from FFY 1989 through 1995. The study population consists of all short-stay hospitals (approximately 5,000) participating in Medicare and receiving case payments by diagnosis-related groups. STUDY DESIGN: The original cost functions used to develop indirect medical education payment adjustments under PPS are re-estimated with panel data. Specification changes are included based on findings from critiques of the original hospital cost model. Additional variations on the model are explored to test for differences by hospital status, to control for the effect of additional disproportionate share and outlier payments, and to isolate the effects of improved case-mix measurement on model results. PRINCIPAL FINDINGS: Fixed effects regression produces no evidence of a significant within-hospital association between increased sponsorship of medical residents and increased cost per case. In models designed to capture a cross-sectional association, operating costs are positively related to teaching activity, but the association shows a decline in strength over time. In all years, the strength of the association is significantly greater among hospitals eligible for disproportionate share adjustments and among major teaching hospitals. Controlling for secular trends of increased teaching intensity results in a pattern of declining cross-sectional teaching coefficients that supports a theory that observed teaching effects are the result of unmeasured case severity. CONCLUSIONS: A significant but declining cost differential is observed between teaching and nonteaching hospitals. The association appears to be related to hospital and patient characteristics that cannot be controlled using currently available case-mix and wage indices. Longitudinal models do not provide evidence to support a payment adjustment formula that allows individual hospitals to recompute their IME adjustment rates as their teaching ratios rise or fall from year to year. Cross-sectional findings suggest that re-estimations of the teaching effect may be appropriate when significant improvements occur in Medicare case-mix measurement.