IRS collecting revenue

This area contains statistics on tax collections by type of tax, as well as by State, for over 30 years. Additional data are also available here.
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SOI Tax Stats - IRS Data Books: 2003 - 2001

Look here for the 2001 - 2003 IRS Data Books (Publications 55B), with links to the 1995 to 2000 IRS Data Books. The tables shown in these publications contain statistics for IRS collections and refunds, as well as number of returns. There's also information on examination activities, taxpayer assistance, chief counsel, appeals, criminal investigation, and more.
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Tax Professionals

SOI tax stats - Individual high income tax returns: returns with and without U.S. Income tax and with income of $200,000 or more under alternative concepts

Individual High Income Tax Returns Statistics: Returns With and Without U.S. Income Tax and With Income of $200,000 or More Under Alternative Concepts: Tables 3, 5, and 7.
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SOI Tax Stats - Nonfarm sole proprietorship statistics

Nonfarm Sole Proprietorship returns bulletin article and related tables.
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SOI tax stats - Historic Table 2 (2010-2014)

Statistics of Income, Historic Table 2 for 2010-2014
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Tax Professionals