The investigators concluded that approximately 500,000 taxpayers who visited the centers during the course of the study, from July to December 2002, could have received incorrect responses to their tax law questions.

IRS employees provided complete and correct answers to 45 percent of the questions asked by auditors, and correct but incomplete answers in 12 percent of the cases.

IRS employees told the auditors to do their own research in IRS publications to find the answers in response to 12 percent of the questions, despite an IRS policy banning the practice.

Incorrect answers were given to 28 percent of the questions. The questions most commonly answered incorrectly dealt with the earned income tax credit, education credit and dependents.

The IRS disputed the calculations, but agreed the agency needed to improve its record. Henry O. Lamar, commissioner for the division overseeing individual tax returns, said the accuracy rate is closer to 67 percent when the results are recalculated to exclude instances when taxpayers were referred to other publications or could not get any help.

"We recognize that an accuracy rate of 67 percent for tax law service is inadequate," he said.

The IRS has taken steps to teach employees more about tax law, implementing continuous education and training programs. The IRS set a goal to give accurate responses to 80 percent of the questions this year and 85 percent next year.

There are 500 taxpayer assistance centers throughout the United States.

-- Mary Dalrymple, AP Tax Writer