The goal is to reward quality, penalize poor performance, and avoid paying piecemeal for services. Whether it succeeds or fails, it's one of the biggest changes in Medicare's 50-year history.

The complex regulation is nearly 2,400 pages long and will take years to fully implement. It's meant to carry out bipartisan legislation passed by Congress and signed by President Barack Obama last year.

The concept of paying for quality has broad support, but the details have been a concern for some clinicians, who worry that the new system will force small practices and old-fashioned solo doctors to join big groups. Patients may soon start hearing about the changes from their physicians, but it's still too early to discern the impacts.

The Obama administration sought to calm concerns Friday. "Transforming something of this size is something we have focused on with great care," said Andy Slavitt, head of the federal Centers for Medicare and Medicaid Services.