New research from Stanford University has found that many online advertising companies continue to track users' Web activity even when they've opted out, reports the San Jose Mercury News. The research has prompted renewed calls from advocates for a do-not-track law. The study found that out of 65 online advertising companies, half continued tracking after the user had opted out. One privacy group has asked the FTC to investigate eight of those online companies. Some companies have since revised their privacy policies with regard to opt-out tracking. Meanwhile, one start-up is launching a system allowing users to block online companies that don't honor do not track. 
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