United States. Bureau of Labor Statistics. "Coal," in United States. Bureau of Labor Statistics. "February 1987," PPI Detailed Report (February 1987) : 14-16. https://fraser.stlouisfed.org/title/63/item/21615/toc/390125 , accessed on March 28, 2024.
United States. Bureau of Labor Statistics. "Coal," in United States. Bureau of Labor Statistics. "February 1987," PPI Detailed Report (February 1987) : 14-16. https://fraser.stlouisfed.org/title/63/item/21615/toc/390125 , accessed on March 28, 2024.