Medicare enrollees tend to flock to one to three Medicare Advantage carriers in each market.

Researchers at the Henry J. Kaiser Family Foundation, Menlo Park, Calif., have published that conclusion in a report based on analysis of federal Medicare Advantage enrollment data.

In 14 states and the District of Columbia, one carrier enrolls more than half the Medicare Advantage enrollees, researchers say.

In most other states, two or three carriers cover a majority of the enrollees.

New York state is the only state in which the top three carriers cover fewer than half of the Medicare Advantage enrollees, researchers say.

Medicare Advantage plans now cover 11 million of the 47 million Medicare enrollees.

The Medicare Advantage program gives private nonprofit and for-profit carriers a chance to sell subsidized health coverage to Medicare enrollees. Most program carriers try to provide the kinds of benefits that enrollees would get if they combined traditional Medicare coverage with Medicare supplement insurance but at a lower total price.

Program supporters, including America’s Health Insurance Plans, Washington, say Medicare Advantage plans provide services that improve enrollees’ quality of life and may improve their life expectancy; critics say the program is too expensive.