Table 2. Pearson's Correlation Coefficient Matrix of Data from Three Influenza Surveillance Systems by Surveillance Year: Google Flu Trends, CDC Influenza-like Illness Surveillance, CDC Influenza Virologic Surveillance, September 28, 2003 through May 17, 20081.
| Surveillance Year | Dataset | Google Flu Trends | CDC ILI | CDC Virologic |
| 2003–04 | Google Flu Trends | 1.00 | -- | -- |
| CDC ILI | 0.94 (0.89, 0.97) | 1.00 | -- | |
| CDC Virologic | 0.67 (0.43, 0.82) | 0.84 (0.69, 0.92) | 1.00 | |
| 2004–05 | Google Flu Trends | 1.00 | -- | -- |
| CDC ILI | 0.98 (0.95, 0.99) | 1.00 | -- | |
| CDC Virologic | 0.94 (0.89, 0.97) | 0.94 (0.88, 0.97) | 1.00 | |
| 2005–06 | Google Flu Trends | 1.00 | -- | -- |
| CDC ILI | 0.97 (0.94, 0.99) | 1.00 | -- | |
| CDC Virologic | 0.72 (0.50, 0.85) | 0.79 (0.62, 0.89) | 1.00 | |
| 2006–07 | Google Flu Trends | 1.00 | -- | -- |
| CDC ILI | 0.94 (0.89, 0.97) | 1.00 | -- | |
| CDC Virologic | 0.71 (0.49, 0.85) | 0.81 (0.64, 0.90) | 1.00 | |
| 2007–08 | Google Flu Trends | 1.00 | -- | -- |
| CDC ILI | 0.98 (0.96, 0.99) | 1.00 | -- | |
| CDC Virologic | 0.91 (0.82, 0.95) | 0.92 (0.85, 0.96) | 1.00 |
1
Because CDC surveillance is intensified from calendar week 40 through calendar week 20 of the subsequent year, we restricted our correlation analyses to this time period.