The Centers for Medicare and Medicaid Services (CMS) administers Medicare, the primary US health insurance program for people aged 65 years and older and people who qualify for Social Security Administration disability benefits (Box). Medicare includes Part A, which is hospital insurance; Part B, medical insurance; Part C, Medicare Advantage (private health insurance approved by the CMS and paid on a per-capita basis); and Part D, prescription drug coverage. The CMS maintains and makes several data files available for purchase (). Because the Medicare Advantage data are administered by private insurance, these claims are unavailable. However, national fee-for-service Medicare files are available and represent approximately 70% of beneficiaries.