I came in expecting something like an Amazon shopping history. I don't find it too surprising that in-person purchases can be tracked so readily - geographic in

I came in expecting something like an Amazon shopping history. I don't find it too surprising that in-person purchases can be tracked so readily - geographic information is a huge dimension to add to these data. It would be interesting to see how many data points are necessary to achieve similar accuracy just with a purchase history. Particularly interesting would be matching users across websites.