I assume the underlying code is a secret, but I’m interested in the basic theory for how iTunes uses the cumulative ratings of millions of user’s songs in the music library to recommend appropriate songs in a similar genre.
ITunes’ approach seems fundamentally different form Netflix’s recommendation which assumes that users who have a large number of similar likes and dislikes in common will have similar taste in unrated movies. Unlike previous recommendation algorithms, the genre of the movie is unimportant; it won’t suggest the “western” Blazing Saddles just because the user liked Shane and The Searchers.
With, iTunes, however, the genre, or at least type of song (loud, rap, oldie, etc.) is more important. If I select Peter, Paul and Mary’s “Leaving on a Jet Plane,” iTunes’ Genius recommends Bob Dylan’s “Blowing in the Wind.” How does it know that the two songs are musically a good match just because many people may have rated them the same? I’ve also rated a lot of hard rock, alternative and grunge songs highly. Why doesn’t it also suggest Nirvana’s “Smells Like Teen Spirit” since I’ve given it five stars, too. I’m sure I’m not the only one with all three of those songs both A) in my music library and B) with five star ratings.