In its drive to reduce Medicare spending, the federal government is trying to increase participation in accountable care organizations by aligning patients with a particular ACO if their primary care doctor is part of one.

The change comes in response to complaints from doctors about the current system, in which the administration assigns a certain number of a doctor’s patients to the ACO retroactively, meaning the ACO is not being judged based on all of the patients it treats. That system, say officials, was aimed at preventing doctors from cherry-picking the healthiest patients to participate in the program.

Under the new system, doctors will know from the beginning which patients’ treatment will be assessed as part of the Medicare Shared Savings program, in which roughly 480 ACOs are currently participating on a voluntary basis.