(Bloomberg) — The Obama administration will make historic changes to how the United States pays its annual $3 trillion health care bill, aiming to curtail a costly habit of paying doctors and hospitals without regard to quality or effectiveness.

Starting next year Medicare, which covers about 50 million elderly and disabled Americans, will base 30 percent of payments on how well health providers care for patients, some of which will put them at financial risk based on the quality they deliver. By 2018, the goal is to put half of payments under the new system.

For doctors and health facilities, the system will tie tens, and then hundreds, of billions of dollars in payments to how their patients fare, rather than how much work a doctor or hospital does, lowering the curtain on Medicare's system of paying line-by-line for each scan, test and surgery.