Yields on actively traded non-inflation-indexed issues adjusted to constant maturities. The 30-year Treasury constant maturity series was discontinued on February 18, 2002, and reintroduced on February 9, 2006.
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), 30-Year Treasury Constant Maturity Rate [GS30], retrieved from FRED, Federal Reserve Bank of St. Louis https://research.stlouisfed.org/fred2/series/GS30, May 24, 2016.