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Google tracking cookie spans AdSense, DoubleClick

Um, but it keeps the data separate?

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Yes, Google is using the same tracking cookie across both its AdSense and DoubleClick online ad contraptions. This allows the Mountain View ad giant to collect your surfing habits as you move from AdSense partner sites to sites using DoubleClick's ad management platform - although the company indicates that at least in some cases, the data is not combined.

"Although we use the same cookie, our data use rights vary per product according to our contracts with our customers," company spokeswoman Christine Chen told us over email.

It's unclear which contracts and which customers Chen is referring too. And although the company says that at least some contracts do not allow it to merge data, it's unclear how the data is separated. If the data is separated as is, it would be trivial to combine it.

Despite our repeated inquires via phone and email, the company has not directly addressed these issues.

Earlier this week, three graduate students in the School of Information at the University of California, Berkeley uncloaked a new study showing that Google AdSense is used by over 35 per cent of a nearly-400,000-site cross-section of the web and that DoubleClick is used by over 26 per cent. According to the study, the next most prevalent web tracker is Omniture, with 6 per cent.

In a story detailing the study The New York Times has Google counsel Mike Yang saying that Google’s contracts with customers do not allow it to merge data from various services like DoubleClick and AdSense. Google verified the statement with us, but did not say whether customer contacts disallow the merging of all AdSense and DoubleClick data or how the company separates such data.

Even if Google isn't merging data now, what's to prevent it from merging that data in the future - i.e. what's to prevent it from changing those mystery contracts? The contracts aren't with web surfers. They're with website owners or advertisers.

The ambiguity cuts to the heart of the ongoing complaints over Google's attitude towards privacy. In a recent pitch to Capitol Hill staffers, Google insists that none of the privacy fears over its 2007 merger with DoubleClick were ever realized. But the company won't give the world the information it needed to determine whether those fears were justified or not.

In March, Google announced what it calls an "interest-based advertising" program, saying it would begin using user web behavior to target ads on YouTube and the many third-party sites in its AdSense network. The company pointed out that user behavior would be collected via cookie technology fashioned by DoubleClick. But it didn't publicly address whether users would be tracked across DoubleClick sites sites as well.

Google does let you opt out of its DoubleClick cookie. But most users aren't even aware they're being tracked. And those who are aware have no way of knowing how extensively they're being tracked. ®

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