United States. Bureau of Labor Statistics. "Coal," in United States. Bureau of Labor Statistics. "December 1988," PPI Detailed Report (December 1988) : 18-19. https://fraser.stlouisfed.org/title/63/item/21637/toc/231918 , accessed on April 23, 2024.
United States. Bureau of Labor Statistics. "Coal," in United States. Bureau of Labor Statistics. "December 1988," PPI Detailed Report (December 1988) : 18-19. https://fraser.stlouisfed.org/title/63/item/21637/toc/231918 , accessed on April 23, 2024.