United States. Bureau of Labor Statistics. "Business Sector - Appendix - Quarterly Data," in United States. Bureau of Labor Statistics. "Second Quarter 2001 [Preliminary]," Productivity and Costs (August 7, 2001) : 12-12. https://fraser.stlouisfed.org/title/131/item/55964/toc/445981 , accessed on April 19, 2024.