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Merger and acquisition activity in the health insurance industry threatens to further erode market competition, which many benefit insiders feel is already dismal. Recent research from the American Medical Association identified the top 10 states that have experienced the biggest drop in competition levels between 2010 and 2013 based on market concentration levels as determined by the Herfindahl–Hirschman Index or HHI—a measure used by the U.S. Department of Justice and the Federal Trade Commission.

The HHI number can range from close to zero to 10,000. DOJ considers a market with a result of less than 1,000 to be a competitive marketplace; a result of 1,000-1,800 to be a moderately concentrated marketplace; and a result of 1,800 or greater to be a highly concentrated marketplace.