United States. Bureau of Labor Statistics. "Coal," in United States. Bureau of Labor Statistics. "March 1988," PPI Detailed Report (March 1988) : 14-16. https://fraser.stlouisfed.org/title/63/item/21628/toc/410032 , accessed on April 19, 2024.
United States. Bureau of Labor Statistics. "Coal," in United States. Bureau of Labor Statistics. "March 1988," PPI Detailed Report (March 1988) : 14-16. https://fraser.stlouisfed.org/title/63/item/21628/toc/410032 , accessed on April 19, 2024.