Source: Standard And Poor'S Corporation, Data For 1900-1928: Standard Statistics Co., Base Book; Standard Statistical Bulletin, January 1932, P. 125. Data For 1929-1967: Computed By NBER By Taking A Simple Average Of Four Series, All Based On Standard And Poor'S Data -- Series 13023 (Municipal Bonds); Series 13024 (Railroad Bonds); Series 13025 (Public Utilities); Series 13026 (Industrial Bonds).
This NBER data series m13021 appears on the NBER website in Chapter 13 at http://www.nber.org/databases/macrohistory/contents/chapter13.html.
NBER Indicator: m13021
Release: NBER Macrohistory Database
National Bureau of Economic Research, Index of Yields of High Grade Corporate and Municipal Bonds for United States [M13021USM156NNBR], retrieved from FRED, Federal Reserve Bank of St. Louis https://research.stlouisfed.org/fred2/series/M13021USM156NNBR/, July 30, 2015.