United States. Bureau of Labor Statistics. "Coal," in United States. Bureau of Labor Statistics. "June 1988," PPI Detailed Report (June 1988) : 14-16. https://fraser.stlouisfed.org/title/63/item/21631/toc/199972 , accessed on April 19, 2024.
United States. Bureau of Labor Statistics. "Coal," in United States. Bureau of Labor Statistics. "June 1988," PPI Detailed Report (June 1988) : 14-16. https://fraser.stlouisfed.org/title/63/item/21631/toc/199972 , accessed on April 19, 2024.