Federal researchers reported on Tuesday that the number of Americans without health insurance had declined substantially in the first quarter of this year, the first federal measure of the number of uninsured Americans since the Affordable Care Act extended coverage to millions of people in January.

The number of uninsured Americans fell by about 8 percent to 41 million people in the first quarter of this year, compared with 2013, a drop that represented about 3.8 million people and that roughly matched what experts were expecting based on polling by private groups, like Gallup. The survey also measured physical health but found little evidence of change.

The findings were part of the National Health Interview Survey, a nationally representative examination that is considered a gold standard by researchers. It interviewed about 27,000 people in the first quarter, fewer than Gallup, which interviewed 45,000 people in the second quarter alone. But researchers say it is considered particularly trustworthy because federal interviewers conduct the survey in Americans’ homes. It also sets a federal level that others can use as a benchmark.

Larry Levitt, a director at the Program for the Study of Health Reform and Private Insurance at the Kaiser Family Foundation, a health research organization, said the first-quarter findings “dramatically understate the effect” of the law, as almost half of the people who signed up for insurance during the open enrollment period did so in March and did not get their insurance cards until later. Private surveys have shown that there were eight million to 10 million fewer uninsured by the second quarter, he said.

Regardless of what you think of the A.C.A., there should be no doubt at this point that the law is increasing the number of people insured,” he said.

Katherine Baicker, a professor of health economics at the Harvard School of Public Health, said of the finding: “That sounds reasonably consistent with what had been expected.”

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Jeffrey R. Ungvary President

Jeffrey R. Ungvary