United States. Bureau of Labor Statistics. "Coal," in United States. Bureau of Labor Statistics. "April 1987," PPI Detailed Report (April 1987) : 23-25. https://fraser.stlouisfed.org/title/63/item/21617/toc/254221 , accessed on April 25, 2024.
United States. Bureau of Labor Statistics. "Coal," in United States. Bureau of Labor Statistics. "April 1987," PPI Detailed Report (April 1987) : 23-25. https://fraser.stlouisfed.org/title/63/item/21617/toc/254221 , accessed on April 25, 2024.