Quite a bit, apparently. Data anonymizing doesn’t necessarily guarantee your privacy.
When the authors mapped locations, dates, and prices of someone’s non-anonymous purchases against the whole database, it was usually easy to find a single, unique pattern. With three points or more, it was virtually a certainty. […] There’s a 94 percent chance that you’re the only person who did so. Taking away price altogether made these matches harder to find. But with four purchases, it was back up to 90 percent.