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Look here for the 2004 - 2005 IRS Data Book (Publications 55B), with links to the 1995 to 2000 and 2001 to 2003 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.

SOI Tax Stats - Corporation Source Book Statistical Tables 2010 All Sectors