United States. Bureau of Labor Statistics. "Coal," in United States. Bureau of Labor Statistics. "April 1988," PPI Detailed Report (April 1988) : 14-17. https://fraser.stlouisfed.org/title/63/item/21629/toc/416867 , accessed on May 5, 2024.
United States. Bureau of Labor Statistics. "Coal," in United States. Bureau of Labor Statistics. "April 1988," PPI Detailed Report (April 1988) : 14-17. https://fraser.stlouisfed.org/title/63/item/21629/toc/416867 , accessed on May 5, 2024.