OPM said the increases were necessary because law requires that rates be sufficient to cover the expected cost of benefits, and some of the original assumptions underlying the premium and benefit structure proved to be incorrect–including the returns on program investments and the share of people who drop out after enrolling. OPM said the number of enrollees did not play a role in the premium increases. There are about 224,000 current enrollees, out of an estimated eligible population of some 20 million federal employees, retirees and certain family members and similarly situated persons in the military community.