WASHINGTON_It's a central goal of the president's plan: Extending health care coverage to the millions of Americans who lack it. Question is, just how many million are uninsured?

The answer could make a huge difference in the billions of dollars it will cost to remake the national system.

Barack Obama frequently cites last year's Census Bureau number of 46 million people with no health insurance. But some experts argue that figure is off by tens of millions _ in one direction or the other.

The recession's continuing toll on jobs, a tendency to undercount people on Medicaid and other factors make it hard to come up with an exact number. And the most widely accepted range _ 40 million to 50 million _ includes some 10 million non-citizens, a detail that's generally overlooked when Obama and others talk about "uninsured Americans."

The lack of certainty about such big numbers is one more question mark for Obama and members of Congress as they try to craft a plan that would cover most of the uninsured. Obama says his goal is to cover 97 percent to 98 percent of Americans, a target that would be reached by plans taking shape in the Senate _ if you don't count illegal immigrants. A bill crafted by House Democrats comes in closer to 94 percent.

All the plans would exclude illegal immigrants, who account for as much as 17 percent of the uninsured, according to the Pew Hispanic Center.

"I want to cover everybody," Obama said at a news conference last month. "Now, the truth is that unless you have a what's called a single-payer system in which everybody is automatically covered, then you're probably not going to reach every single individual."

Some people don't want health insurance or just don't bother to get it, but most people who don't have it can't afford it, Obama said.

"So I think that the basic idea should be that in this country, if you want health care, you should be able to get affordable health care," he said.

New Census Bureau figures expected next month could scramble the equation, adding billions in costs if the numbers come in higher than expected, or reducing costs if the numbers are lower.

There could be serious implications "if we all of a sudden found that instead of 45 million uninsured there are 35 million," said Michael O'Grady, a senior fellow at the University of Chicago's health policy and evaluation department and a former assistant secretary at the Department of Health and Human Services.


A lower figure could cut two ways: making Congress' job cheaper, but also making the country's health care woes seem less pressing.

Even if there are fewer uninsured than now estimated, health experts emphasize that it's still a lot of people, and being uninsured has consequences. The Institute of Medicine has found that uninsured people are more likely to succumb to illness and suffer premature death.

Still, some overhaul foes are accusing the media of overreporting the number of uninsured in order to frighten the public and "bolster calls for universal government-run insurance coverage," as a report by the conservative media watchdog Media Research Center's Business and Media Institute put it.

The 46 million number (actually 45.7 million) cited by Obama and others comes from the Census Bureau's annual Current Population Survey for 2007. It's the consensus figure, but some researchers believe the CPS overstates the number of uninsured people, partly by undercounting how many people are on Medicaid, the federal-state program for the poor.

Another government survey, the Medical Expenditure Panel Survey done by the Department of Health and Human Services, says that about 40 million people were uninsured for all of 2007, and about 70 million were uninsured for part of the year.

All those numbers are out-of-date. Taking into account the effects of the recession, with widespread job losses cutting into employer-provided health care _ more than 5 million jobs have been lost since last August _ researchers at the Urban Institute and elsewhere estimate that the present-day number of uninsured is closer to 50 million. That's the number used by the Congressional Budget Office.

The Census Bureau is releasing its Current Population Survey for 2008 on Sept. 10. Then, later in September, for the first time, it's releasing health coverage information collected by the American Community Survey, which has a much larger sample size than the CPS. Some researchers are expecting that number to be more precise.

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On the Net:

U.S. Census Bureau Current Population Survey: http://www.census.gov/cps/

Medical Expenditure Panel Survey: http://www.meps.ahrq.gov/mepsweb/