United States. Bureau of Labor Statistics. "Coal," in United States. Bureau of Labor Statistics. "December 1987," PPI Detailed Report (December 1987) : 19-21. https://fraser.stlouisfed.org/title/63/item/21625/toc/116127 , accessed on March 28, 2024.
United States. Bureau of Labor Statistics. "Coal," in United States. Bureau of Labor Statistics. "December 1987," PPI Detailed Report (December 1987) : 19-21. https://fraser.stlouisfed.org/title/63/item/21625/toc/116127 , accessed on March 28, 2024.