When you buy from a large company like Target, you're not just making a transaction. You're adding one more data point to what Target calls your Guest ID, a number that's associated not only with purchases made with a credit card or using a coupon, but with a list of demographic information collected or purchased from an outside agency. That Guest ID, however, isn't just used to help you find things you already know you want. Instead, it's being used to predict when customers will be more open to shopping at Target, and what they might want to buy — even if they don't know it yet.

Building on existing research about habit formation, Target's Guest Marketing Analytics department looks at times when people are open to shaking up their purchasing practices, such as marriages or pregnancies, and leverages these into coupons or advertisements that subtly suggest products that will get them shopping at Target. If a woman buys enough of roughly 25 "pregnancy indicator" products like unscented lotion or vitamins, for example, Target can tell not only that she's likely pregnant, but also roughly what trimester she's in and tailor coupons accordingly. While she's shopping for items for the new baby, she's also more likely to pick up things that she'd normally buy at other retailers.

Like much commercial data mining, the results are sometimes unsettling. In one case, a teenage girl started receiving coupons for cribs and maternity clothes in the mail. The father, irate, came to Target to complain — only to find out a few weeks later that his daughter was, in fact, pregnant. Although we're sure the pregnancy wasn't going to stay a secret forever, it's also probably not the only case of purchase histories being inadvertently outed to family members or roommates though targeted ads. At the same time, the process of collecting and analyzing this information is nothing short of fascinating. Be sure to click through to the source link below for more about how Target and other companies have succeeded (and failed) at knowing you better than almost anyone else.