For the primary analysis, scatter plots with least square regression lines were constructed to compare Google Flu Trends and CDC ILI Surveillance to the standard reference surveillance (CDC Virus Surveillance). Pearson's correlation coefficients with 95% confidence intervals (95% CI) were then computed from these comparisons. Subsequently, additional correlation coefficients with 95% CI were calculated from surveillance comparisons by influenza season, US Census Region, and influenza season categorized by US Census Region. These subset analyses were for the non-specific complaint of influenza-like illness (ILI) based on internet key word searches. 2 CDC Influenza-like Illness Surveillance involves a network of health care providers who record the weekly proportion of patients seen with ILI. Google Flu Trends was created and validated using CDC ILI Surveillance data, explaining the similarity between the two curves. 3 CDC Influenza Virologic Surveillance consists of about 140 laboratories located throughout the United States that report the weekly total specimens tested and laboratory tests positive for influenza virus. This is the only US surveillance system that provides national and regional data of laboratory-confirmed influenza virus infection. 4 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. doi:10.1371/journal.pone.0018687.g001 summarized with mean correlation coefficients and standard deviations (SD). Next, because Flu Trends was previously found to lead CDC ILI Surveillance observations by one to two weeks [2] , we undertook additional correlation analyses to determine whether Google Flu Trends or CDC ILI Surveillance had a stronger correlation with CDC Virus surveillance data for the subsequent one or two weeks. The unit of analysis was percentage of clinic patients with ILI (CDC ILI Surveillance and Google Flu Trends) and percentage of laboratory tests positive for influenza (CDC Virus Surveillance).