[howlin]

There are a lot of ways of formalizing the problem of recommendation. Perhaps the variant of the problem used by Netflix is "solved", but it's kind of an odd one. Basically, they built a system to answer questions of the following form: "Given that user X watched media Y, what rating would they give it?" They trained and tested on media that users have already rated. Some of the ratings are masked and thus need to be "predicted" for the test.

The issue is that the Netflix dataset has a baked-in assumption that a recommender system should show media that a user is likely to have ranked highly. It may be more important to show the user media they wouldn't have found (and thus ranked) at all. Or perhaps a user will be more engaged with something controversial rather than generically acceptable. Who knows?