Washington ? IRS tax-return audit rates increased slightly last year, almost entirely among low-income taxpayers who filed simple returns, according to a study released today.

Audit rates were down for taxpayers with incomes of more than $100,000, continuing a decadelong decline in examinations of the returns of the wealthiest Americans, according to an analysis of Internal Revenue Service data by the Transactional Records Access Clearinghouse at Syracuse University.

Today is the deadline for Americans to file income tax returns.

Audit rates for low-income taxpayers, which first exceeded those for the wealthy in 1999, did so again last year. The IRS says that is because of a congressional mandate that it aggressively examine claims for a low-income tax credit. Most of those audits are conducted with letters requesting more information rather than through face-to-face meetings.

Overall, the odds of being audited were about 1 in 170, compared with 1 in 200 the previous year.

The clearinghouse said taxpayers least likely to be audited were those with incomes between $25,000 and $100,000. For them, the audit rate was less than in one in 400.

“Particularly striking” over the past 10 years has been the decline in audit rates for the wealthy, said Susan Long, a Syracuse University professor and co-director of the clearinghouse.

High-income taxpayers remain more likely to be audited face to face than lower-income taxpayers, though the odds of any taxpayer being required to submit to such an audit are quite small.

Last year, 0.16 percent of all taxpayers were audited face-to-face, including 0.08 percent of those with simple returns and income of less than $25,000 and 0.38 percent of those with incomes of more than $100,000, the study found.