The American Medical Association published its annual study on competition in commercial health insurance markets.
Market concentration in all 384 metropolitan statistical areas, the 50 states, and Washington, D.C., was analyzed using the Herfindahl-Hirschman Index. Federal guidelines classify markets with an index score of more than 2,500 points as "highly concentrated."
Fifteen states where one health insurer held statewide market share of at least 50 percent:
1. Alabama
2. Michigan
3. Hawaii
4. South Carolina
5. Delaware
6. Kentucky
7. Louisiana
8. Illinois
9. Indiana
10. Mississippi
11. North Carolina
12. Oklahoma
13. North Dakota
14. Vermont
15. Alaska
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