Não informado. (2004). Ers. U.S. Department of Agriculture, Economic Research Service.
Chicago-referens (17:e uppl.)Não informado. Ers. Washington: U.S. Department of Agriculture, Economic Research Service, 2004.
MLA-referens (8:e uppl.)Não informado. Ers. U.S. Department of Agriculture, Economic Research Service, 2004.
Varning: dessa hänvisningar är inte alltid fullständigt riktiga.