Averages of business days. For information on the break in this series from January 1987 through September 1993, please refer to information available at http://www.federalreserve.gov/releases/h15/data.htm.
For further information regarding treasury constant maturity data, please refer to http://www.federalreserve.gov/releases/h15/current/h15.pdf and http://www.treasury.gov/resource-center/data-chart-center/interest-rates/Pages/yieldmethod.aspx.
Release: H.15 Selected Interest Rates
Board of Governors of the Federal Reserve System (US), 20-Year Treasury Constant Maturity Rate [GS20], retrieved from FRED, Federal Reserve Bank of St. Louis https://research.stlouisfed.org/fred2/series/GS20/, August 4, 2015.