When unveiling its search-data-driven Flu Trends modeler earlier this week, Google insisted it could never be used to identify the web habits of individual people. Flu Trends, the company said, uses nothing but "anonymized" data.

Of course, no one knows what that means.

Following the release of the new disease tracker, two concerned watchdogs - the tech-minded Electronic Privacy Information Center (EPIC) and the medical-minded Patient Privacy Rights - tossed a letter at Google CEO Eric Schmidt, asking for an explanation. "Would you agree to publish the technique that Google has adopted to protect the privacy of search queries for Google Flu Trends?" the letter asked. "As you know, there is considerable debate as to what constitutes 'anonymized' data."

Famously, when AOL released "anonymized" data into the wild back in the summer of 2006, the world quickly realized it wasn't that anonymous. EPIC and Patient Privacy would like to know if Google has somehow succeeded where AOL failed.

"We're asking Google: 'If you've solved the problem of how to take individual search histories and provide some useful aggregate data without creating a risk of re-identification, let's see it, let's discuss it, let's use it as a basis for solving some very hard privacy problems," EPIC president Marc Rotenberg told The Reg.

Google's Trends service has long used aggregated search data to track the habits of the world's web users. But health-related data is a particularly touchy subject, and Rotenberg sees Flu Trends as a chance to broaden the public debate over data aggregation - and finally put some meaning into these anonymization claims.

The problem, Rotenberg says, is that data aggregation calls attention to specific data stored on Google's servers, making it that much more vulnerable to, say, a subpoena or a national security letter. "Let's say that instead of Flu Trends, Google's doing SARS Trends - tracking a very serious communicable disease," he explains. "If there's a big SARS upsurge somewhere, the government would be at Google's door asking where did that data come from."

And that's just one example. "You can imagine any number of different scenarios where people would be interested in finding who the individuals are making those searches." In his letter to Schmidt, Rotenberg points to precedent. "Census data, the quintessential form of aggregate data, was used during the Second World War to identity and then displace Japanese Americans," the letter reads. "The Department of Homeland Security sought information from the US Census about Muslim Americans in the United States after 9-11."

Schmidt hasn't responded to the letter. And after we asked about the letter, Google hasn't responded to us. But the world will keep knocking.

"This privacy issue is genuine and it needs to be solved," Rotenberg tells us. "Google even concedes that in saying that their data is anonymized and aggregated. That's great if it's true. But how do we know it's true?" ®