United States. Bureau of Labor Statistics. "Coal," in United States. Bureau of Labor Statistics. "May 1988," PPI Detailed Report (May 1988) : 14-16. https://fraser.stlouisfed.org/title/63/item/21630/toc/151348 , accessed on June 24, 2024.
United States. Bureau of Labor Statistics. "Coal," in United States. Bureau of Labor Statistics. "May 1988," PPI Detailed Report (May 1988) : 14-16. https://fraser.stlouisfed.org/title/63/item/21630/toc/151348 , accessed on June 24, 2024.