We evaluated geographic variation at the following 3 levels on the basis of the beneficiary's zip code of residence: 306 HRRs, 50 states and the District of Columbia, and 4 national regions (South, West, Midwest, and Northeast). Hospital referral regions were defined by where patients were referred for major cardiovascular surgical procedures and for neurosurgery and are often used as proxies for regional health care markets. Many programs have aimed to improve appropriate antibiotic use and to reduce inappropriate use in ambulatory care settings. Because some of these programs represent efforts as a result of policy at the state level, we also examined variation in antibiotic use across states. Finally, we attempted to link quarterly data with the aggregated level of regional data for the prevalence of some conditions. These 4 areas were defined using the US Census region definitions, which are as follows: the South (Alabama, Arkansas, Delaware, District of Columbia, Florida, Georgia, Kentucky, Louisiana, Maryland, Mississippi, North Carolina, Oklahoma, South Carolina, Tennessee, Texas, Virginia, and West Virginia), the West (Alaska, Arizona, California, Colorado, Hawaii, Idaho, Montana, Nevada, New Mexico, Oregon, Utah, Washington, and Wyoming), the Midwest (Illinois, Indiana, Iowa, Kansas, Michigan, Minnesota, Missouri, Nebraska, North Dakota, Ohio, South Dakota, and Wisconsin), and the Northeast (Connecticut, Maine, Massachusetts, New Hampshire, New Jersey, New York, Pennsylvania, Rhode Island, and Vermont).