This suggested that Medicare Advantage didn’t provide the type of coverage or the access to services that unhealthier beneficiaries wanted or needed. Since the point of insurance is to pay for needed care when one is sick, it was tempting to condemn the program as having poor quality and failing to fulfill a basic requirement of coverage.

But things have changed. Mr. Newhouse and Mr. McGuire show, for example, that by 2006-2007, health differences between beneficiaries in Medicare Advantage and those in traditional Medicare had narrowed. About the same proportion of beneficiaries in Medicare Advantage as in traditional Medicare rated their health as fair or poor. This suggests that sicker beneficiaries were not switching out of Medicare Advantage and healthier ones were not switching in to the extent they had been in earlier years.

Also, in contrast to studies in the 1990s, more recent work finds that Medicare Advantage is superior to traditional Medicare on a variety of quality measures. For example, according to a paper in Health Affairs by John Ayanian and colleagues, women enrolled in a Medicare Advantage H.M.O. are more likely to receive mammography screenings; those with diabetes are more likely to receive blood sugar testing and retinal exams; and those with diabetes or cardiovascular disease are more likely to receive cholesterol testing.

That Health Affairs paper also found that H.M.O. enrollees are more likely to receive flu and pneumonia vaccinations and about as likely to rate their personal doctor and specialists highly.

There are reasons Medicare Advantage plans might promote higher-quality care. So long as beneficiaries don’t switch among plans too rapidly (and the evidence is that once they select a plan, they tend to stick with it), plans have a financial incentive to keep their enrollees healthy, incurring less downstream cost. It’s possible, therefore, that they may offer incentives to providers to perform preventive services.