United States. Bureau of Labor Statistics. "Coal," in United States. Bureau of Labor Statistics. "May 1987," PPI Detailed Report (May 1987) : 14-16. https://fraser.stlouisfed.org/title/63/item/21618/toc/132857 , accessed on April 24, 2024.
United States. Bureau of Labor Statistics. "Coal," in United States. Bureau of Labor Statistics. "May 1987," PPI Detailed Report (May 1987) : 14-16. https://fraser.stlouisfed.org/title/63/item/21618/toc/132857 , accessed on April 24, 2024.