The Medicare Advantage program has grown from almost 7 million people, or 17 percent of beneficiaries in 2000, to about 16 million people in 2014, or 30 percent of beneficiaries.

Under Medicare Advantage, Medicare pays private insurance plans a predetermined rate per person instead of paying for every medical treatment or service a patient gets.

Ninety-nine percent of Medicare beneficiaries live in an area where a private Medicare Advantage plan is offered.

Between 2013 and 2014, enrollment in Medicare Advantage plans grew by about 9 percent, or 1.3 million beneficiaries, according to a March 2015 report. About 30 percent of Medicare beneficiaries were enrolled in MA plans in 2014, up from 28 percent in 2013.

Medicare will spend about 2 percent more in 2015 on payments for people in private plans than for people who use the traditional fee-for-service program, MedPAC projected in its March report. But the plans also tell Medicare that their patients are sicker on average than people in the traditional fee-for-service program, and Medicare pays the plans more as a result.

This year, Medicare program will pay about $8 billion more toward the care for people in Medicare Advantage than it would have spent if those people stayed in fee-for- service Medicare.

Medicare spent about $159 billion on Medicare Advantage last year.

Among the companies with the largest enrollment in Medicare Advantage are UnitedHealthcare, Humana, Blue Cross and Blue Shield affiliated companies, Kaiser Permanente, Aetna, Wellcare, and Cigna.