United States. Bureau of Economic Analysis. "Gross Domestic Product (Third Estimate), Corporate Profits, and GDP by Industry, Fourth Quarter and Year 2020 : Technical Note, BEA 21-11," Bureau of Economic Analysis (BEA) News Releases (March 25, 2021). https://fraser.stlouisfed.org/title/6148/item/601651/content/pdf/bea_newsreleases_20210325_technote, accessed on March 18, 2025.