We used the 2000 MarketScan Research Database (MEDSTAT, Ann Arbor, Mich), which consists of medical care claims, prescription drug claims, encounter data, and health plan benefit descriptions from approximately 45 large employers, health plans, and government and public organizations. Each year of the data set contains information on approximately 2 million active and retired employees. The encounter files cover data on age, sex, and geography of residence. Prescription drug claims include the national drug codes, date of purchase, metric quantity, days supply for each drug, and expenditure information, including total payments, out-of-pocket payments made by patients, and net payments made by the employer. Health care claims contain the same financial information as well as the date of service, diagnosis, procedure codes, and type of provider. These data can be linked to a health plan data set that contains details on the benefit as abstracted from plan booklets. Plan information includes the dates of enrollment, type of insurance plan (eg, comprehensive plans, preferred provider organizations, health maintenance organizations), mail-order pharmacy options, generic incentives, and descriptions of the prescription copayments, including differential amounts for preferred and nonpreferred branded drugs.