Medicare is a national social insurance program, administered by the U.S. federal government, that guarantees access to health insurance for Americans ages 65 and older and younger people with disabilities as well as people with end stage renal disease. As a social insurance program, Medicare spreads the financial risk associated with illness across society to protect everyone, and thus has a somewhat different social role from private insurers, which must manage their risk portfolio to guarantee their own solvency.[1] Medicare offers all enrollees a defined benefit. Hospital care is covered under Part A and outpatient medical services are covered under Part B. To cover the Part A and Part B benefit Medicare offers a choice between an open-network single-payer plan (traditional Medicare)and a network plan (Medicare Advantage, or Medicare Part C), where the federal government pays for private health coverage. A majority of Medicare enrollees have traditional Medicare (76 percent) over a Medicare Advantage plan (24 percent). Medicare Part D covers outpatient prescription drugs exclusively through private plans, either standalone prescription drug plans or through Medicare Advantage plans that offer prescription drugs In 2010, Medicare provided health insurance to 48 million Americans—40 million people age 65 and older and eight million younger people with disabilities. Medicare serves a large population of old, sick, and low-income people, many of whom would be unable to afford health care otherwise. On average, Medicare covers about half (48 percent) of health care costs for enrollees. Medicare enrollees must cover the rest of the cost. These out-of-pocket costs vary depending on the amount of health care a Medicare enrollee needs. They might include uncovered services—such as long-term, dental, hearing, and vision care—and supplemental insurance.[2]