People are (rightly) concerned about who has access to their personal information theses days. But even when you avoid social networking sites altogether, data about you is still being collected.

Every time you go shopping, you share intimate details about your consumption patterns with retailers. And many of those retailers are studying those details to figure out what you like, what you need, and which coupons are most likely to make you happy. Target, for example,…assigns every customer a Guest ID number, tied to their credit card, name, or email address that becomes a bucket that stores a history of everything they’ve bought and any demographic information Target has collected from them or bought from other sources.

That’s from Kashmir Hill over at Forbes, telling the unsettling story of how—through data mining and statistical analysis—Target ended up knowing a teen girl was pregnant before her father did. (The story originates with Charles Duhigg, the author of The Power of Habit.  His story “How Companies Learn Your Secrets” is a must read over that the NYT Magazine.)