Among the key findings of the study:

On average, Marketplace enrollees had access to 40% of the doctors near their home through their plan's network, with considerable variation around the average. Twenty-three percent of Marketplace enrollees were in a plan with a network that included a quarter or fewer of the doctors in their area, while only 4% were in a plan that included more than three-quarters of the area doctors in their network.
Some of the narrowest network plans were found in large metro counties, where enrollees on average had access to 34% of doctors through their plan networks. Marketplace enrollees in Cook County, Ill (Chicago) and Lee County, Fla. (Fort Myers) were enrolled in some of the narrowest networks (with average physician participation rates of 14% and 23%, respectively).
Plans in rural counties tended to include a larger share of the doctors in the area, although rural counties had fewer doctors overall relative to the population compared to large metro counties.
On average, more than one-quarter of actively practicing physicians were not included in any Marketplace plan network.

Silver plans with higher shares of participating doctors had higher total premiums on average. Compared to plans where 25% or fewer of doctors participated in-network, those with participation rates between 25% and 50% cost 3% more, while those with participation rates of more than 50% cost 8% more. Silver plans are particularly significant. because they are the benchmark for federal premium subsidies.
More than four million enrollees (37% of all enrollees) lived in a county in which the two lowest-cost Silver plans included fewer than half of the doctors in the area and a broader plan was available. For these enrollees to enroll in the least expensive Silver plan that included at least half the doctors, they would have needed to spend an additional $88 per month.

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Size of physician networks varies widely among ACA Marketplace plans, study finds