Retrospective and prospective performance of original and updated Google Flu Trends (GFT) algorithm compared with national (United States), regional (Mid-Atlantic States) and local (New York City) weekly influenza-like illness (ILI) surveillance data, 2003–2013.

Performance was evaluated by linear regression of weekly GFT estimates against weekly ILI surveillance.
aOriginal GFT model time periods: The retrospective query selection model-fitting period was from September 28, 2003 through March 17, 2007; the prospective GFT model validation period was from March 18, 2007 through May 17, 2008 and ongoing operation was from May 18, 2008 through Aug 1, 2009. Mid-Atlantic region states included NJ, NY and PA (13). New York comparison was based on NY state GFT estimates (16).
bUpdated GFT model time periods: the retrospective query selection model-fitting period was from September 28, 2003 through September 18, 2009; The prospective operation period has run from September 19, 2009 through March 30, 2013. Mid-Atlantic region states included only NJ and NY (14). The New York level comparison was based on New York City GFT estimates (16).