In early 2012, a story broke about a man who had come into a Target store wielding a fistful of coupons addressed to his high-school-aged daughter. He had come to berate the manager because this set of coupons were exclusively for things such as baby clothes, formula, and furniture.

On hearing the man's complaint, the manager apologized profusely. In fact, he felt so bad that he wanted to follow up several days later over the phone to explain how this had happened. Only this time, on the phone, the father was the one apologizing. It seems his daughter was, in fact, pregnant. Her shopping habits had given her away.

The algorithm that gave her away was most likely based, at least in part, on collaborative filtering.

So, what is collaborative filtering?

Collaborative filtering is based on the idea that out there somewhere you have a taste doppelganger. The assumption is that you have rated a certain set of items in a way that is very similar to the way this doppelganger...