Source: U.S. Department of the Treasury. Internal Revenue Service  

Release: SOI Tax Stats - Historical Data Tables  

Units:  Number of Returns, Not Seasonally Adjusted

Frequency:  Annual

Represents the sum of (a) "estimated tax," generally paid quarterly throughout the tax year by self-employed and certain other taxpayers with insufficient or no tax withheld, and (b) prior-year tax overpayments taxpayers elected to credit to their current-year's "estimated tax" in lieu of requesting a refund.

For additional information about the return filing requirements, see the annual reports, Statistics of Income-Individual Income Tax Returns at https://www.irs.gov/uac/soi-tax-stats-historical-data-tables.