Many companies offer users the chance to opt-out of having their movements tracked on websites, but not all of them keep their promise to not monitor online activity.
Advertising networks and publishers store information of user behaviour online in 'cookies': a small text file of user activity stored on a user's computer that websites can access. Companies use these cookies to help target adverts based on user behaviour.
The Network Advertising Initiative (NAI) runs a voluntary scheme that encourages businesses to comply with certain rules about monitoring user activity online.
They stipulate that companies should inform users of their intent to store information in cookies, and that the information would be used to serve behavioural adverts. They must also stop using cookies if the user wishes.
A team of researchers from the Stanford Law School carried out three tests on NAI members to see if they were abiding by the rules.
One of the researchers, Jonathan Mayer, said: "At least eight NAI members promise to stop tracking after opting out, but nonetheless leave tracking cookies in place."
While some of these companies deleted cookies once users had opted out, the researchers found that sometimes the files were restored after users revisited the website. Other companies would delete some of the cookies but not all.
One of the eight firms that promised to stop tracking after opting out did not delete any cookies when asked to.
Of all the NAI member companies, more than half of them kept their cookies in place even though the researchers (posing as users) asked to opt out.
The researchers said: "Of the 64 companies we studied, 33 left tracking cookies in place after opting out."
NAI member companies need only give users the chance to opt out of having behavioural advert targeting, but they may still have tracking cookies stored.
Of the 64 companies involved, at least ten, one of these being Google, deleted both tracking and behavioural advert information.
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