Item-based filtering

If a group of people have rated two items similarly, then the two items must be similar. Therefore, if a person likes one particular item, they're likely to be interested in the other item too. This is the principle on which item-based filtering works. Again, Amazon makes good use of this model by recommending products to you based on your browsing and purchase history, as shown in the following screenshot:

Item-based filters, therefore, recommend items based on the past ratings of users. For example, imagine that Alice, Bob, and Eve have all given War and Peace and The Picture of Dorian Gray a rating of excellent. Now, when someone buys The Brothers Karamazov, the system will recommend War and Peace as it has identified that, in most cases, if someone likes one of those books, they will like the other, too.