In Reply: We would like to offer 3 clarifications about the comments from Dr Phillips and colleagues regarding our recent study. First, the study's multilevel (ie, hierarchical) models used the patient as the unit of analysis with patient covariates at the individual, not area (ie, ecological), level. Physician workforce was included in the model at an area level, reflecting that population-based physician workforce measures are a contextual concept.
Second, the study population was not limited to those who used Part A or Part B services. The Methods stated, “We used a 20% national sample of fee-for-service Medicare beneficiaries and analyzed 100% of their physician and hospital claims. Beneficiaries were included if they resided in the United States, were aged 65 to 99 years on January 1, 2007, and had Part A (acute care in facilities, including hospitals) and Part B (clinician services) coverage in 2007 (N = 5 132 936).” That is, we studied beneficiaries who were eligible for Part A and Part B service payments (ie, with Part A and Part B Medicare coverage), whether or not they received care.