J.L.: Profiling care provided by different groups of physicians: effects of patient case-mix (bias) and physician-level clustering on quality assessment results

cian-level variation (clustering) are often overlooked in profiling the quality of care provided by different groups of physicians, such as specialties. Objective: To examine the effect of case-mix bias and physi-cian-level clustering on differences in quality of diabetes care between specialty groups participating in the American Diabetes Association’s Provider Recognition Program. Design: Retrospective record review of both process and out-come measures over 1 year and a cross-sectional patient survey. The sample included 29 solo and group practice sites in diverse regions of the United States. Of the 29 sites, 15 were endocri-nology sites and 14 were primary care sites. Patients: 1750 adults with diabetes. Measurements: Process measures included frequency of hemo-globin A1c, lipid, and urine protein testing; blood pressure mea-surement; and foot and eye examinations. Outcome measures included A1c level, blood pressure, lipid levels, and patient satisfaction. Patient case-mix variables included age, sex, health status, level of education, ethnic minority status, and duration of diabetes. Results: Unadjusted differences between endocrinologists and generalists were statistically significant for most process and out-come measures. Inclusion of patient case-mix variables reduced the statistical significance of specialty differences for some quality measures. After accounting for the substantial physician-level clustering, observed differences between specialties were no longer statistically significant for any of the quality measures except patient satisfaction. Conclusions: The findings underscore the importance of design-ing physician profiling studies with sufficient power to account for physician-level variation (clustering) as well as patient case-mix. Studies that are not designed with both sufficient numbers of physicians and patients per physician may distort differences in quality of care between physician groups