Some web sites still track users despite behavioural ad opt-out, say researchers

Publishers and advertising networks use cookies to track user behaviour on websites in order to target adverts to individuals based on that behaviour. A cookie is a small text file that websites store on users' computers to remember their activity on a site.

Researchers at Stanford Law School investigated whether companies belonging to a voluntary scheme run by the Network Advertising Initiative (NAI) actually complied with the rules they had signed up to.

The NAI encourages online businesses to voluntarily adopt a set of rules governing online behavioural advertising. Those rules force member companies to tell users that cookies they store about them could be used to serve behavioural ads. The rules also state that member companies must stop using the cookies to serve ads if asked to by users.

The researchers claimed that at least 8 NAI members out of the 64 they investigated continued placing behavioural ad cookies on researchers' machines after being asked not to.

"At least eight NAI members promise to stop tracking after opting out, but nonetheless leave tracking cookies in place," Jonathan Mayer, one of the Stanford researchers, said in a blog.

Mayer said that the researchers had performed three tests of the NAI members' cookie promises. In some of the cases the companies did delete the cookies but the researchers found that they were restored after they revisited content on the website, Mayer said in his blog. In other cases some of companies deleted some cookies and not others, he said.

One of the eight firms, Vibrant Media, did not delete any of the tracking cookies when asked, the researchers claimed.

More than half of NAI members tested left tracking cookies in place after researchers had told websites they were opting out. Researchers said that NAI members only have to allow users to opt out of behavioural ad targeting and not tracking.

"Of the 64 companies we studied, 33 left tracking cookies in place after opting out," the researchers said.

At least 10 NAI members, including Google, delete tracking cookies as well as behavioural ad targeting, the researchers said.

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