United States. Bureau of Labor Statistics. "Industry Wage Survey: Bituminous Coal Mining—Part I, Underground Mines; Part II, Surface Mines, January 1967 : Bulletin of the United States Bureau of Labor Statistics, No. 1583," Industry Wage Survey: Bituminous Coal (1968). https://fraser.stlouisfed.org/title/4582/item/498403, accessed on January 2, 2025.