United States. Bureau of the Census. "Diffusion Indexes," in United States. Bureau of the Census. "March 1969," Business Conditions Digest (March 1969) : 94-96. https://fraser.stlouisfed.org/title/43/item/7528/toc/62174 , accessed on April 25, 2024.
United States. Bureau of the Census. "Diffusion Indexes," in United States. Bureau of the Census. "March 1969," Business Conditions Digest (March 1969) : 94-96. https://fraser.stlouisfed.org/title/43/item/7528/toc/62174 , accessed on April 25, 2024.