Managing the medical costs of ACA populations requires a deep understanding of the utilization dynamics within unique member cohorts. New Oliver Wyman analysis shows that a typical ACA population includes a cohort of members whose health and utilization patterns will be similar to Commercial Group Business. However, within the same product set, about 20 to 30 percent of members will pattern more like Medicare members. In fact, the costliest segment of an ACA population can be even higher than a Medicare Advantage (MA) population. Our analysis indicates that the top 1 percent of an ACA plan can drive between 20 and 30 percent of total spend.