Federal health officials are doubling down on the stability of the ObamaCare insurance marketplace, touting a new report that confirms a greater mix of younger, healthier people have enrolled.

The data show that the average medical costs for each person with an ObamaCare plan stayed about flat between 2014 and 2015, compared to a 3 percent increase among customers in the wider market.

{mosads}That trend shows that more healthy people signed up for ObamaCare between its first enrollment period and its second, according to the Centers for Medicare and Medicaid Services (CMS), which runs the federal marketplace.

Still, the scope of the new report is limited: It covers only the enrollment periods ending in 2014 and 2015, when healthier people were already expected to sign up in greater numbers.

Experts had anticipated more healthy people would sign up in 2015, after the initial influx of sicker people, many of whom had been barred from coverage because of preexisting conditions.

Marilyn Tavenner, the president of the America’s Health Insurance Plans trade group, called the report “an overly optimistic assessment.”

“The reality is that the risk pool has not significantly improved. That is a serious concern,” Tavenner wrote in a statement.

The CMS said the new report offers the deepest look yet at the mix of customers in the marketplace. Dr. Mandy Cohen, chief operating officer for the agency, called it “encouraging and exciting.”  

“Available evidence implies that the slow [Affordable Care Act] individual market cost growth results at least in part from a broader, healthier risk pool,” according to the report.