United States. Bureau of Labor Statistics. "Coal," in United States. Bureau of Labor Statistics. "June 1987," PPI Detailed Report (June 1987) : 14-16. https://fraser.stlouisfed.org/title/63/item/21619/toc/45775 , accessed on April 18, 2024.
United States. Bureau of Labor Statistics. "Coal," in United States. Bureau of Labor Statistics. "June 1987," PPI Detailed Report (June 1987) : 14-16. https://fraser.stlouisfed.org/title/63/item/21619/toc/45775 , accessed on April 18, 2024.