According to a recent study:

• 80.4% of Medicare beneficiaries who were assigned to an Accountable Care Organization (ACO) in 2010, were assigned to the same ACO in 2011, and 19.6% were not • Among ACO-assigned beneficiaries, 8.7% of office visits with primary care physicians were provided outside of the assigned ACO • Among ACO-assigned beneficiaries, 66.7% of office visits with specialists were provided outside of the assigned ACO • 37.9% of Medicare spending on outpatient care billed by ACO physicians was devoted to ACO-assigned beneficiaries, and 62.1% was not Source: "Outpatient Care Patterns and Organizational Accountability in Medicare," JAMA Internal Medicine, abstract only, April 21, 2014, https://archinte.jamanetwork.com/article.aspx?articleid=186103