United States. Bureau of Labor Statistics. "Handbook of Labor Statistics, 1931 edition : Part 1 : Bulletin of the United States Bureau of Labor Statistics, No. 541,"
Handbook of Labor Statistics
(September 1931).
https://fraser.stlouisfed.org/title/4025/item/493215/content/pdf/bls_0541_1931pt1, accessed on August 12, 2026.