United States. Bureau of Labor Statistics. "Footnotes," in United States. Bureau of Labor Statistics. "Private Business, Nonfarm Business, and Manufacturing Sectors : Second Quarter 1981," Productivity and Costs (July 30, 1981) : 12-12. https://fraser.stlouisfed.org/title/131/item/55906/toc/437962 , accessed on April 25, 2024.