United States. Bureau of Economic Analysis. "Gross Domestic Product, 4th Quarter and Year 2020 (Advance Estimate) : Technical Note, BEA 21-02," Bureau of Economic Analysis (BEA) News Releases (January 28, 2021). https://fraser.stlouisfed.org/title/6148/item/600347/content/pdf/bea_newsreleases_20210128_technote, accessed on May 15, 2025.