United States. Bureau of Labor Statistics. "Diffusion Indexes of Employment Change," in United States. Bureau of Labor Statistics. "Employment, Hours, and Earnings, United States, 1909-90 : Bulletin of the United States Bureau of Labor Statistics, No. 2370," Employment and Earnings, United States (March 1991) : 240-241. https://fraser.stlouisfed.org/title/189/item/5435/content/pdf/emp_bmark_1909_1990_v2/toc/421325 , accessed on May 6, 2024.