Correlations among influenza surveillance systems also differed by US Census Region. The correlation coefficients describing the association between Google Flu Trends and CDC Virus Surveillance over the study period ranged from 0.64 (95% CI 0.54, 0.72) in the East North Central Region to 0.80 (95% CI 0.74, 0.85) in the West North Central Region ( Table 3 ). The mean correlation coefficient for these comparisons was 0.70 (SD 0.05). The correlation between CDC ILI Surveillance and CDC Virus Surveillance ranged from 0.64 (95% CI 0.55, 0.73) in the East South Central Region to 0.86 (95% CI 0.81, 0.89) in the West South Central Region. The mean correlation coefficient for these comparisons was 0.76 (SD 0.07).