The U.S. Census Bureau is altering the questions it asks about health insurance so fundamentally that it will be hard to weigh the impact of President Obama's health care law against previous year's findings.

The changes, first reported by the New York Times, are taking place just as the American public is weighing Obamacare's effects and trying to figure out the net number of people who have gained insurance versus those who have had their plans canceled in the wake of the new law.

A test run with the new questionnaire produced lower estimates of the uninsured than in previous years, which could result to overestimates of the Affordable Care Act's impact on increasing the numbers of those with insurance nationwide.

The Times reported that the Department of Health and Human Services and the White House Council of Economic Advisers requested many of the new questions, and the White House Office of Management and Budget approved the new questionnaire.

The Obama administration has reported that 7.5 million people signed up for insurance on state and federal exchanges by the March 31 deadline and that enrollment in Medicaid has increased by three million since October.

Officials so far have said they don't have estimates of the number of enrollees who actually paid their first month's premium or how many young healthy people have signed up for the plan, a key measure of the new law's success.

Concrete information has been scarce about those who are signing up for the law and how it is affecting the level of uninsured across the country. The timing of the census overhaul has Republicans crying foul.

“How terribly convenient,” remarked Brendan Buck, a spokesman for Speaker John Boehner, R-Ohio, in an email to reporters.

Republicans on the House Energy and Commerce Committee sent out the story to reporters in an email with the subject line: “Accounting of Newly Insured Gets Murkier.”

Census Bureau Director John H. Thompson defended the timing of the new questions, saying they are based on 14 years of research and two national tests conducted in 2010 and 2013.

The change was announced in September and "implemented because the evidence showed that reengineering the questions provides demonstrably more accurate results," he said. "The Census Bureau only implements changes in survey methodology based on research, testing and evidence presented for peer review."

The revised questions were put in the Census Bureau's Current Population Survey starting in February and will cover 2013. Therefore, he said, the new questionnaire will "provide a more accurate baseline for assessments of changes in insurance coverage, including that of the Affordable Care Act."

This article was originally published at 5:14 p.m. and has been updated.