Source: P.H. Douglas, Real Wages In The United States, 1890-1926, P. 382; Douglas And Jennison, The Movement Of Money And Real Earnings In The United States, 1926-1928, P. 25, University Of Chicago Studies In Business Administration, No. 3.
This NBER data series a08059 appears on the NBER website in Chapter 8 at http://www.nber.org/databases/macrohistory/contents/chapter08.html.
NBER Indicator: a08059
Release: NBER Macrohistory Database
National Bureau of Economic Research, Average Annual Earnings of Teachers for United States [A08059USA052NNBR], retrieved from FRED, Federal Reserve Bank of St. Louis https://research.stlouisfed.org/fred2/series/A08059USA052NNBR/, September 1, 2015.