Did you ever consider how your eating habits are tracked by that little barcoded "shopping club" thing they scan at the supermarket checkout? They ring up all those items and then you give them a little plastic card with a barcode to scan. Now they have a name to go with all those items including over-the-counter drugs, and magazines. If you use the supermarkets pharmacy and use that card, they also know what prescriptions you take and can thus deduce some of your medical condition. The magazines you buy tell them what your hobbies are.