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Google Flu Trends

Google has a Flu Trends project where you can see influenza trends across the world estimated by search queries.
Here’s an example from my state, Pennsylvania:

Flu Information

Get more information on protecting yourself and keeping your family healthy during the flu season.

Prevention

http://www.cdc.gov/flu/protect/preventing.htm

Vaccine Finder

http://flushot.healthmap.org/

Other Reporting/Analysis Sites

HealthMap Flu Near You

How Does Google Do It?

I found myself wondering what search terms they are using to evaluate flu trends.  It was actually quite interesting to read the article published by Nature.com.  If you’re not a member of Nature.com, you can still download the article from Google in PDF format.
Reading over the article, I looked at the following table to see how they weighted queries from search data.

2013-01-18_151020
Topics found in search queries which were found to be most correlated with CDC ILI data. The top 45 queries were used in our final model; the next 55 queries are presented for comparison purposes. The number of queries in each topic is indicated, as well as query volume-weighted counts, reflecting the relative frequency of queries in each topic.

Google, in the article, states the following:

Because the relative frequency of certain queries is highly correlated with the percentage of physician visits in which a patient presents with influenza-like symptoms, we can accurately estimate the current level of weekly influenza activity in each region of the United States, with a reporting lag of about one day.