United States. Bureau of Labor Statistics. "Coal," in United States. Bureau of Labor Statistics. "March 1987," PPI Detailed Report (March 1987) : 14-16. https://fraser.stlouisfed.org/title/63/item/21616/toc/410779 , accessed on April 20, 2024.
United States. Bureau of Labor Statistics. "Coal," in United States. Bureau of Labor Statistics. "March 1987," PPI Detailed Report (March 1987) : 14-16. https://fraser.stlouisfed.org/title/63/item/21616/toc/410779 , accessed on April 20, 2024.