Forty percent of the most popular type of health plan sold on insurance exchanges offer consumers only a limited choice of doctors, in some cases less than 10 percent of physicians practicing in the area, according to research released Wednesday.

These "narrow network" plans place limits on the doctors and hospitals available to their enrollees, often excluding high-cost providers. As a result, the premiums for such plans are lower, and attractive to price-sensitive consumers.

Narrow networks existed long before the Affordable Care Act. But they have been the focus of consumer criticism and some regulatory scrutiny now that most Americans are required to have insurance. Many enrollees have been frustrated by the lack of up-to-date information about doctors in their network, and by surprise bills from providers they thought were in their network, but turned out not to be.

"When consumers are choosing a plan, they should know what they're getting," said Kathy Hempstead, who directs coverage issues at the Robert Wood Johnson Foundation, which funded the research by the University of Pennsylvania's Leonard Davis Institute of Health Economics. "There's a tradeoff between premium price and breadth of their [provider] network."

[Consumer backlash over hospitals excluded from plan network]

Right now, consumers can find out if specific doctors are in their plan's network. But they have no idea, she said, if those doctors represent 10 percent or 40 percent of all the practicing physicians in that specialty.

"That's a missing piece of information that consumers deserve to have," she said. If consumers had that information, they might be able to comparison-shop and choose to pay a higher premium to have a network that includes more doctors.

The research is the first to analyze physician provider networks in plans sold on the marketplaces. A report on hospital networks by McKinsey & Co. found that 39 percent of networks in plans sold on the marketplaces were "narrow," defining them as having fewer than 70 percent of hospitals in a rating area.

The University of Pennsylvania researchers collected data on providers in networks tied to silver plans--the most popular type of coverage--sold in all 50 states and the District. Their analysis looked at more than 450,000 physicians participating in plans issued by 251 insurance carriers across 355 provider networks.

They found that 41 percent of doctor networks are small or extra-small: 11 percent of networks are extra-small, meaning they have less than 10 percent of office-based practicing doctors in the area and another 30 percent are small, meaning they have between 10 percent and 25 percent of all physicians.

Within specialty groups, 36 percent of primary care doctor networks were small or extra-small, and nearly 60 percent of the networks of oncologists were small, according to the findings.

Within different plan types, such as PPO or HMO, the size of the doctor networks also varied, the researchers found. Most doctor networks are either PPO or HMO. With most HMO plans, a consumer's health-care services are coordinated with a designated primary care doctor. In a PPO plan, consumers can choose their own doctors from within the network.

The researchers found that 55 percent of HMOs had small or extra-small doctor networks, meaning consumers had the least amount of choice, compared to 25 percent of PPOs.