People, like Mr. Murphy, who earn too much to qualify for federal subsidies that defray the cost of coverage may be most likely to opt out. A recent analysis by the Kaiser Family Foundation found that more than seven million people who are eligible for exchange coverage would pay less in penalties than for the least expensive insurance available to them. More than half would not qualify for subsidies, the analysis found.

Ben Wakana, a spokesman for the Department of Health and Human Services, said people would still be better off buying insurance.

“We understand some people may be thinking through their choice of coverage, but going without health insurance is a serious gamble that can be catastrophic if wrong,” Mr. Wakana said.

Many holdouts have made their decisions after meticulously comparing the cost of insurance premiums and deductibles with paying for doctor appointments, lab tests and prescriptions themselves. For some healthy people, the combined cost of premiums and deductibles, which can exceed $10,000, makes the penalty seem a better deal.

For 2016 and beyond, the penalty will be $695 per adult or 2.5 percent of household income, up from $325 per adult or 2 percent of household income last year.