United States. Bureau of Labor Statistics. "Coal," in United States. Bureau of Labor Statistics. "November 1987," PPI Detailed Report (November 1987) : 15-17. https://fraser.stlouisfed.org/title/63/item/21624/toc/160655 , accessed on May 1, 2024.
United States. Bureau of Labor Statistics. "Coal," in United States. Bureau of Labor Statistics. "November 1987," PPI Detailed Report (November 1987) : 15-17. https://fraser.stlouisfed.org/title/63/item/21624/toc/160655 , accessed on May 1, 2024.