Let's now try to address some of the shortcomings in user-based collaborative filtering with a technique called item-based collaborative filtering, and we'll see how that can be more powerful. It's actually one of the techniques that Amazon uses under the hood, and they've talked about this publicly so I can tell you that much, but let's see why it's such a great idea. With user-based collaborative filtering we base our recommendations on relationships between people, but what if we flip that and base them on relationships between items? That's what item-based collaborative filtering is.