The Internal Revenue Service has cut deeply the time that it spends auditing the nation's largest corporations, according to data made public yesterday.

The figures, obtained by Syracuse University researchers, showed that the I.R.S. had reduced the time spent on each audit by 21 percent in the last five years, to 958 hours from 1,210 hours. At the same time, the number of actual audits, which had increased in the last two years, has fallen back to the level of 2002.

The Syracuse researchers said the data raised questions about statements by Mark W. Everson, the I.R.S. commissioner, that he had increased enforcement of the tax laws in the last three years.

I.R.S. officials said, however, that while the hours devoted to each audit had fallen, the number of additional tax dollars recommended per hour of audit had more than doubled from 2002 to 2006, to $6,500 an hour. The government data is based on fiscal years, which end Sept. 30.

That increase "shows that we are working smarter, we are working more efficiently, we are working more effectively under a tight budget," an I.R.S. spokesman, Terry L. Lemons, said.

David Burnham, a director of the university's Transactional Records Access Clearinghouse, said the higher hourly figure might indicate more effective enforcement, but he proposed another explanation.

"They may just be picking a lot of low-hanging fruit," Mr. Burnham said.

Data that the I.R.S. published this month shows that since 1990, the growth rate in profits reported to shareholders has been more than 50 percent that of profits reported on tax returns, a trend that has happened while Congress has cut the number of tax auditors.

The nation's 12,137 largest corporations, with $250 million or more in assets, earn 87 percent of the profits reported by the nation's more than five million businesses.

About 4,000 of these companies were audited in the fiscal year that ended Sept. 30. If large companies were audited at the same rate as a decade ago, when the auditing staff was a third larger, more than 6,000 of these companies would have had their tax returns examined.

Auditors concluded that the 4,000 companies underpaid the government by $13.7 billion, accounting for three-fourths of all the individual and corporate tax cheating uncovered by I.R.S. examinations of returns that year.

Prof. Sue Long, a director of the Syracuse clearinghouse, said getting the data used in the research had been a problem. The I.R.S., Professor Long said, initially turned over a small number of pages of data in May after Judge Marsha Pechman of Federal District Court in Washington directed the I.R.S. to resume complying with a 1976 order to make the data available. Later, after she complained, the I.R.S. turned over much more data.

Professor Long said the data was a mess. "It took us months to sort it out," she said.

"They told us they had given us all of it, but we noticed from the page numbers in the upper right hand corner that they had not and it turned out they had given us less than 1 percent of the data" that the university researchers eventually received, Professor Long said.

In addition, she said, it is difficult to tell if the data is complete because the I.R.S. will not provide an inventory.

The agency said it provided more than 200,000 pages of data, covering 56 months starting in October 2001, to the Syracuse researchers. It said the reason that only a small number of pages were turned over last May was a misunderstanding about what data was covered by the court order.

Mr. Burnham said that under the previous commissioner, Charles O. Rossotti, the I.R.S. "provided data without requiring a Freedom of Information Act request" while under Mr. Everson litigation has been required to get data.

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