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He also said the enrollment projections are fluid figures that should be viewed with two caveats in mind.

Avalere’s projections reflect those who are expected to enroll through an exchange, but it is possible that some residents will opt to pay a tax penalty for failing to gain insurance — a sanction that kicks in during tax filing season in early 2015 because of the law’s individual mandate.

In that case, the firm’s projected enrollment would fall, Mr. Mendelson said.

Conversely, some of the states listed in the study may adopt the so-called Arkansas model for expanding Medicaid to those making up to 138 percent of the federal poverty level. Under this method, the state uses an influx of federal funds for the entitlement program to purchase private insurance for the newly eligible population, a move that would boost enrollment numbers on the corresponding state exchanges.