See L. Ayres, Turning Points In Business Cycles (New York, Macmillan, 1939) For Data Through 1922 To An Extra Decimal. These Data Were Computed By NBER From Deviations From Trend (100 Plus Or Minus Deviations). Source: Cleveland Trust Company, "American Business Activity Since 1790, " 16Th And 31St Editions For 1855-1970 Data. See Also The Business Bulletin.
This NBER data series m12003 appears on the NBER website in Chapter 12 at http://www.nber.org/databases/macrohistory/contents/chapter12.html.
NBER Indicator: m12003
Release: NBER Macrohistory Database
National Bureau of Economic Research, Index of American Business Activity for United States [M12003USM516NNBR], retrieved from FRED, Federal Reserve Bank of St. Louis https://research.stlouisfed.org/fred2/series/M12003USM516NNBR, February 13, 2016.