United States. Bureau of Labor Statistics. "Coal," in United States. Bureau of Labor Statistics. "January 1988," PPI Detailed Report (January 1988) : 18-20. https://fraser.stlouisfed.org/title/63/item/21626/toc/174735 , accessed on April 18, 2024.
United States. Bureau of Labor Statistics. "Coal," in United States. Bureau of Labor Statistics. "January 1988," PPI Detailed Report (January 1988) : 18-20. https://fraser.stlouisfed.org/title/63/item/21626/toc/174735 , accessed on April 18, 2024.