United States. Bureau of Economic Analysis. "U.S. International Trade in Goods and Services: March 2018 : Full Release, BEA 18-21, CB 18-69," Bureau of Economic Analysis (BEA) News Releases (May 3, 2018). https://fraser.stlouisfed.org/title/6148/item/602628/content/pdf/bea_newsreleases_20180503, accessed on May 16, 2025.