In the three months since their release, the initial results of Medicare’s Pioneer Accountable Care Organization (ACO) program have generated divergent interpretations by analysts and policymakers. Some have pointed to savings and quality improvements in the first year as evidence that the program is off to a promising start to improving the value of care. Others cite the nine organizations leaving the program and the humbling results of a prior ACO experiment in Medicare — the Physician Group Practice Demonstration (PGPD) that ran from 2005 to 2010 — as reasons to be pessimistic.