The findings highlight the difficult balancing act the administration faces in carrying out one of the the health-care law’s most sweeping, yet ambiguous, mandates. The statute sets out 10 general categories — ranging from hospitalization to prescription drugs — that all new insurance plans for individuals and small businesses must offer starting in 2014. It also states that the scope of the essential benefits package should be equal to that of a “typical employer plan.”

But Congress did not specify whether this referred to the more generous plans sponsored by large employers or the more minimalist versions bought by many small businesses. And it gave Secretary of Health and Human Services Kathleen Sebelius ultimate authority to decide both how much more detailed to make the package and what to include in it.

If she adds little to the legal requirement, the market could end up split between cheap, bare-bones plans of use only to the healthy, and exorbitantly priced full coverage plans financially out of reach of many sick people who need them most.

If she adds too many requirements, premiums for all plans could soar — with consequences for not just individuals but the success of the law as a whole. That’s because many healthy people could decide to pay a penalty instead of buying pricey insurance, skewing the risk pool toward the sick and causing premiums to spiral higher.