- Who Pays Taxes in America? (PDF). Citizens for Tax Justice. 12 April 2012.
- Webster (3): decreasing in rate as the base increases (a regressive tax)
- American Heritage 互联网档案馆的存檔，存档日期2008-06-03. (3). Decreasing proportionately as the amount taxed increases: a regressive tax.
- Dictionary.com (3).(of tax) decreasing proportionately with an increase in the tax base.
- Britannica Concise Encyclopedia: Tax levied at a rate that decreases as its base increases.
- Sommerfeld, Ray M., Silvia A. Madeo, Kenneth E. Anderson, Betty R. Jackson (1992), Concepts of Taxation, Dryden Press: Fort Worth, TX