The Congressional Budget Office on Friday acknowledged the challenges it has in projecting how repealing Obamacare would affect insurance coverage.

The nonpartisan agency on Friday released a presentation on how it figures out the impact of Obamacare's individual mandate on coverage, a few days after a new Obamacare overhaul bill was announced that seeks to repeal the law's individual and employer mandates. The CBO is reviewing the bill and is expected to issue an estimate of its effect on insurance coverage and the deficit in the coming weeks.

Prior CBO estimates of Obamacare repeal bills found that repealing the mandate could lead to as many as 16 million people going without insurance.

Republicans repeatedly criticized the CBO's estimates, saying the mandate has been ineffective in getting people to sign up for Obamacare. The mandate requires people to buy insurance or pay a penalty.

The CBO's presentation referred to a study released in May that showed the response to the mandate could grow over time. But the study, co-authored by one of Obamacare's architects, Jonathan Gruber, found that overall coverage rates weren't affected as much by the individual mandate.

The CBO said it is difficult to figure the impact of repealing the mandate because repealing it is "not the same as never having had a mandate."

"How much will the knowledge about the benefits of having health insurance, subsidies and the enrollment process that consumers have already gained affect their decisions in the future?" the presentation asked.

The Internal Revenue Service, which collects the mandate penalty, found that 58 percent of the people who pay the mandate earn less than $50,000. About 28 percent earn up to $99,999, and 14 percent earn $100,000 or more.

The IRS collects the mandate penalty when tax returns are filed.

The individual mandate requires every American to have health insurance or face a fine. There are exemptions for people who are too poor to pay the fine, which in 2016 was $695 or 2.5 percent of household income.

The CBO said the mandate's penalty wasn't meant to be the only way to force people to sign up for health insurance. Other factors include that people tend to comply with laws and people could respond more to penalties than to subsidies.

But the mandate hasn't worked out as intended. Insurers have struggled to sign up younger and healthier people onto Obamacare's exchanges, which led to higher premiums to compensate for the sicker-than-expected population.

The mandate was intended to be a key "stick" to get younger people to sign up, with premium tax credits the "carrot" to lower the cost and entice participation.

The presentation comes after Republicans have called for more transparency on how CBO makes its estimates.

Sen. Bill Cassidy, R-La., one of four co-sponsors of the new Obamacare overhaul bill, cast doubt Friday, ahead of the release of the CBO report, about the effectiveness of the individual mandate.

"We know that the individual mandate doesn't work," he said.

He pointed to Gruber's study and read a section in which it said the individual mandate did not have a statistically significant effect in causing gains in health insurance coverage.

"If the CBO dings us because we don't have an individual mandate, the best data out there is that the individual mandate does not increase health insurance," he said.

He noted that he asked insurance commissioners "point blank" last week during hearings on Obamacare about whether it worked.

"They looked at each other and said, 'Not really.' The insurance companies use it to raise rates but then they tell us it doesn't work after all."