Nearly half of the 616 U.S. corporations worth $20 billion or more didn’t have to worry about IRS auditors’ catching mistakes — unintentional or otherwise — on their annual tax returns because federal officials didn’t subject them to reviews, according to data obtained by a nonprofit government watchdog.

Despite a 38 percent increase in the number of such corporations in recent years, IRS auditors went over far fewer of their tax returns than in prior years, according to the Transactional Records Access Clearinghouse (TRAC) at Syracuse University.

The data analyzed by TRAC was obtained under the federal Freedom of Information Act (FOIA).

“These two opposite trends — more returns, yet fewer audits — meant that the odds these corporate giants received an IRS audit fell dramatically. As recently as FY 2010, virtually every (96 percent) corporate giant received an IRS audit,” TRAC said in a report made public Monday.

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“By FY 2016, this had fallen to only three out of four (76 percent). And last year, during FY 2017, the audit rate tumbled to a mere two out of every four — or roughly half (54 percent). This means that nearly half of these corporate behemoths escaped any IRS audit.”

The returns of 331 of the 616 mammoth corporations were audited in 2017, compared to 431 of 447 in 2010.

While it is impossible to know with certainty exactly how much in taxes went unpaid as a result of the unaudited corporate tax returns, TRAC said the total is almost certainly in the multiples of billions of dollars.

“The total additional recommended taxes IRS auditors uncovered in just the few hundred audits of corporate giants turned up more dollars than in all of the remaining corporate audits — of any size — that it conducted last year,” TRAC said.

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“In fact, these 331 audits last year uncovered much more tax underreporting ($10.4 billion) than was turned up in the combined 933,785 audits of tax returns filed by all individuals ($9.0 billion). To borrow from Willie Sutton’s apocryphal rejoinder, clearly that, if the federal government is interested in uncovering underreporting of federal income taxes, it should focus on the returns of corporate giants because that is where the money is.”

Part of the reason for the sharp decrease in such audits is the precipitous decline in the number of available IRS tax agents, according to TRAC. There were 14,749 agents in 2010, compared with 9,605 in 2017.

During the same period, Congress approved — and then-President Barack Obama signed — the Affordable Care Act (aka Obamacare), which mandated extensive additional duties for the IRS, including assessing and collecting billions of dollars in fines for failing to obtain approved health insurance coverage.

Senior editor Mark Tapscott can be reached at [email protected]. Follow him on Twitter.