Table 1. Pearson's Correlation Coefficient Matrix of Data from Three Influenza Surveillance Systems: Google Flu Trends, CDC Influenza-like Illness Surveillance, CDC Influenza Virologic Surveillance, September 28, 2003 through May 17, 20081.
| Dataset | Google Flu Trends | CDC ILI | CDC Virologic |
| Google Flu Trends | 1.00 | -- | -- |
| CDC ILI | 0.94 (0.92, 0.96) | 1.00 | -- |
| CDC Virologic | 0.72 (0.64, 0.79) | 0.85 (0.81, 0.89) | 1.00 |
| CDC Virologic plus One Week | 0.69 (0.60, 0.76) | 0.79 CI: 0.72, 0.84 | -- |
| CDC Virologic plus Two Weeks | 0.66 (0.56, 0.74) | 0.75 (0.68, 0.81) | -- |
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.