WASHINGTON - More than 16 million Americans have gained coverage since President Obama's health-care law took effect five years ago, the administration said Monday. But an independent expert who used some of the same underlying data had the much lower estimate of 9.7 million.

The Department of Health and Human Services estimated that 16.4 million adults have gained health insurance since the law's major coverage provisions began taking effect in 2010. At the same time, data from a large daily survey called the Gallup-Healthways Well-Being Index show that there are 9.7 million fewer uninsured adults now compared with the first part of 2010.

There seems to be no dispute that Obama's law has significantly reduced the number of uninsured Americans.

It will take time for authoritative statistics to emerge, particularly when the government and outside experts are using different estimating methods.

The HHS estimate comes as the administration prepares to commemorate the fifth anniversary of the law's signing on Mar. 23, 2010. The Affordable Care Act has been politically divisive from the start.

HHS Secretary Sylvia M. Burwell said it's brought about "the largest reduction in the uninsured in four decades."

Democrats hailed the law's passage as the culmination of decades of effort to guarantee health coverage for all Americans, including people with health problems who previously could be turned away by insurance companies.

Republicans called it government overreach and haven't stopped trying to repeal or roll back what they dismiss as "Obamacare." That opposition has helped the GOP win control of Congress.

The health-care law offers subsidized private coverage for people who don't have health insurance on the job, along with an expanded Medicaid program that most states have accepted.

According to the HHS estimate, 14.1 million adults got their insurance after the law's big expansion began at the end of 2013. HHS said it used Gallup-Healthways data to arrive at that figure.

Even before that, 2.3 million people had gained coverage under a provision that lets young adults stay on a parent's plan until age 26, the administration said. HHS reached that estimate using data from a different source, a government survey.

The administration says that means 16.4 million people gained coverage under the law.