"Some of these errors may be due to intentionally excessive claims, but the complexity of the rules ... produces large numbers of mistakes even for people trying to get it right," Yale Law School professor Michael Graetz wrote in his recent book The Decline (and Fall?) of the Income Tax.

Simply determining eligibility for the credit requires completion of a 12-question checklist, which is followed by a nine-line worksheet and an eight-line form. All of this requires reference to several pages of Internal Revenue Service instructions.

The IRS, which expects 20 million taxpayers to claim the credit this year, frequently finds up to 40 percent of such returns contain some error. But the people for whom the credit is intended typically don't have the money to hire a tax professional.

"For the most part, people make careless errors or they just don't understand the law," said Candice Cromling, director of the IRS program on the credit.

The earned income tax credit was created in 1975, in part to keep lower-income people working to contribute payroll taxes to Social Security and Medicare and in part to give people incentives to stay off welfare.

Depending on how many children a taxpayer has, people earning generally between $10,030 and $30,095 could be eligible this year. The income levels change each year according to inflation.

The maximum credit is $3,756, and it is refundable, which means it can trigger a refund even for people who don't owe any income taxes. Last year, IRS processed $29 billion in such claims resulting in $23 billion in refunds. Most of the rest resulted in a lower tax bill.

But the Treasury Department estimates 20 percent of people who are eligible never attempt to put in a claim. "A lot of people don't know about it," said Mildred Carter, senior tax analyst at CCH Inc., an Illinois-based consulting firm.

The IRS has begun a campaign to raise people's awareness of the credit, including inserts in envelopes containing W-2 forms, "EITC Days" at community centers and volunteers with churches and charities who help people with their taxes in working-class neighborhoods.

The IRS is also trying to reduce the error rate. Agents screen thousands of returns that claim the credit to check for mistakes as simple as an incorrect Social Security number or faulty math.

But the IRS also reviewed 290,000 suspect returns in 1998 that claimed earned income credits worth $662 million. Of those "flagged" returns, the agency found 68 percent were invalid, according to the General Accounting Office, Congress' investigative agency.

The IRS this year is preventing taxpayers who improperly claimed the credit from attempting to do so again, unless they fill out a new form explaining why they are eligible. For more serious offenders, the IRS can bar the credit for up to 10 years.

There are ways to get help: IRS offices have Publication 596, which explains the law and eligibility rules. Agents at many of those offices can answer questions, or taxpayers can call the agency toll-free at 1-800-829-1040, seven days a week, 24 hours a day.

IRS rules say some taxpayers earning as much as $14,200 and as little as $6,950 a year may not be required to file an income tax return, depending on age, marital status and other factors.

But Cromling said it's worth the effort: The average refund for taxpayers claiming the earned income credit is running at just over $2,000 this year. That compares with a national average refund of about $1,600.