The World Ends. British Phonographic Industry. [2017-02-17] （英语）. Trust, Gary. Ask Billboard: Britney Spears's Career Sales. Billboard. 2015-03-24 [2015-03-24]
Industry Association of America. [2009-07-06]. （原始内容存档于2016-01-07）. Trust, Gary. Ask Billboard: The Twitter-Sized Edition. Billboard. Prometheus Global Media