Medicare is accelerating plans to peg a portion of doctors' pay to the quality of their care. The changes would affect nearly 500,000 physicians working in groups. The federal health law requires large physician groups to start getting bonuses or penalties based on their performance by 2015, with all doctors who take Medicare patients phased into the program by 2017. The program is a major component of Medicare's effort to shift medicine away from its current payment system, in which doctors are most often paid for each service regardless of their performance. The current system, researchers say, financially encourages doctors to do more procedures and is one of the reasons health costs have escalated.