• by Wendy Davis , Staff Writer @wendyndavis, March 27, 2013

Many marketers have made no secret of their desire to target ads to mobile users based on their location.

But a new study indicates that geo-location targeting -- even when theoretically “anonymous” -- raises significant privacy concerns. For the study, researchers at MIT and the Catholic University of Louvain studied fifteen months' worth of "anonymized" data gleaned from 1.5 million people over a 15-month period. From that trove of information, the researchers concluded that “human mobility traces are highly unique.”

The report states: “In a dataset where the location of an individual is specified hourly, and with a spatial resolution equal to that given by the carrier’s antennas, four spatio-temporal points are enough to uniquely identify 95% of the individuals.”

Even when the researchers “coarsened” the data, they still found that people could be identified. “Hence,” the paper continues, “even coarse datasets provide little anonymity.”

That conclusion doesn't mean that geo-location targeting is illegal, only that it's probably not anonymous in any meaningful sense. But any companies who say in privacy policies that they collect anonymous geo-location data collection might need to rethink that language.

Overall, the report offers yet another piece of evidence that individuals can be identified based on “anonymous” data -- a prospect that privacy experts like Paul Ohm have warned of for years.

To some extent, this has already happened. The most famous example occurred in 2006, when AOL released three months worth of search queries for 650,000 users. The searches alone provided clues to some of those users' identities.

That's not the only incident. Several years ago, two computer scientists at the University of Texas reported that it was possible to identify users by comparing reviews of obscure movies on Netflix with reviews on Imdb.com that were published under screennames. Last year, a federal prosecutor was identified as an anonymous commenter at the Times-Picayune's NOLA.com, based on the vocabulary he used in his posts.