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. 2011 Aug 19;6(8):e23610. doi: 10.1371/journal.pone.0023610

Table 2. Correlation and RMSE between United States Google Flu Trends estimates and ILINet data.

Pre-pH1N1(September 2003–March 2009) pH1N1 Overall(March 2009–December 2009) pH1N1 Wave 1(March 2009–August 2009) pH1N1 Wave 2(August 2009–December 2009)
Correlation
Original Model 0.906 0.912 0.290* 0.916
Updated Model 0.942 0.989 0.945 0.985
RMSE
Original Model 0.006 0.018 0.008 0.023
Updated Model 0.005 0.005 0.001 0.007

*The overall correlation during pH1N1 is not an average of the Waves 1 and 2 correlations. The range of ILI rates was larger in Wave 2 than in Wave 1, causing the Wave 2 data to contribute more than the Wave 1 data to the overall correlation during pH1N1.