Dear Global Ad
According to IRS statistics for Tax Year 2002 (scroll down the page)
Earned Income Tax Credit (TY 2002) :
Number of returns with credit (millions) 21.7
Amount claimed (billions of $) $38.2
You'll find another set of numbers here - for IRS's fiscal year 2003,
issued in March of 2004
Earned income tax credit:
Number of returns with credit (millions) 21.3
Amount claimed (billion dollars) 36.9
Number issued (millions) 18.0
Amount (billion dollars)  32.0
There are no published statistics about how many taxpayers
that are entitled to the EIC don't take it. However, let's
look at Individual tax returns filed in 2002 to see who might
Form 1040A with TPI under $25,000 [7,8] 35,631,532
All other returns by size of TPI: 
Under $25,000 18,123,219
You can see there are over 53 million taxpayers who are
apt to qualify, based on low income, especially if they had children.
Only those who are married, or head of household qualify.
(There is a small EIC for single people, but it's insignificant.)
In 2002 the following numbers of returns were filed by
those with a qualified filing status (70 million):
Married, joint - 51,529,487
Head of Household - 19,220,247
I think this is the best indicator, though:
There were a total of 25,975,097 returns that took a Child Tax Credit
(i.e. families with children that might possibly qualify for the EIC)
http://www.irs.gov/pub/irs-soi/02in01si.xls - scroll down to Tax Credits
And 21.3 million taxpayers used the EIC.
So, based on the population of returns with children,
there probably weren't many more than 4 or 5 million
who might have been eligible, who did not use the Earned Income Credit.
Note: The child tax credits are available to families with
incomes much higher than those who qualify for the EIC.
See the income thresholds at which the child tax credits are phased out:
Married Filing Jointly: $110,000
Married Filing Separately: $55,000
Other Filling Status: $75,000
While the Earned Income earnings limits are:
? $33,178 if you have two or more children ($4,140 max. credit)
? $29,201 if you have one child ($2,506 max. credit)
? $11,060 if you have no children ($376 max. credit)
So, it's likely that the number of those who did not claim
the credit is much lower than the 4 or 5 million.
In addition, families with low taxable incomes will also lose
their EIC rights if part of their low income derives from
investments. So that will further reduce the potential
population of those eligible.
One thing that NONE of the IRS statistics show is the number
of people who don't file tax returns at all, who are eligible
for the EIC. And I don't know how to find those number for you.
I hope this helps?