We're now going to cover a topic that's near and dear to my heart-collaborative filtering. Have you ever been to some place like amazon.com and seen something like "people who bought this also bought," or have you seen "similar movies" suggested on imdb.com? I used to work on that. In this section, I'm going to show you some general algorithms on how that works under the hood. Now I can't tell you exactly how Amazon does it, because Jeff Bezos would hunt me down and probably do terrible things to me, but I can tell you some generally known techniques that you can build upon for doing something similar. Let's talk about a technique called item-based collaborative filtering and discuss how that works. We'll apply it to our MovieLens data to actually figure out similar movies to each other based on user ratings.

We're doing some pretty complicated and advanced stuff at this point in the book. The good news is this is probably...