In the Recommender Systems world there is a school of thought that says that it doesn’t matter what type of items you are recommending. For these folks, a recommender is a black box that takes in user behavior data and outputs recommendations. It doesn’t matter what you are recommending – books, music, movies, Disney vacations, or deodorant.  According to this school of thought you can take the system that you use for recommending books and easily repurpose it to recommend music. This is wrong. If you try to build a recommender by taking your collaborative filtering book recommender and applying it to music, you will fail.   Music is different. Music is special.

Here are 10 reasons why music is special and why your off-the-shelf collaborative filtering system won’t work so well with music.

Huge item space – There is a whole lot of music out there. Industrial sized music collections typically have 10 million songs or more.  The iTunes music store boasts 18 million songs.  The algorithms that worked so wonderfully on the Netfix Dataset (one of the largest CF datasets released, contain user data for 17,770 movies) will not work so well when having to deal with a dataset that is three orders of magnitude larger.

Very low cost per item –  When the cost per item is low, the risk of a bad recommendation is low.  If you recommend to me a bad Disney Vacation I am out $10,000 and a week of my time.  If you recommend a bad song, I hit the skip button and move on to the next.

Many item types – In the music world, there are many things to recommend: tracks, albums, artists, genres, covers, remixes, concerts, labels, playlists,  radio stations other listeners etc.

Low consumption time – A book can take a week to read, a movie may take a few hours to watch, a song may take 3 minutes to listen to.  Since I can consume music so quickly, I need lots of recommendations (perhaps 30 an hour) to keep my queue filled,  whereas 30 book recommendations may keep me reading for a whole year.  This has implications for scaling of a recommender. It also ties in with the low cost per item issue.  Because music is so cheap and so quick to consume, the risk of a bad recommendation is very low. A music recommender can afford to be more adventurous than other types of recommenders.

Very high per-item reuse  – I’ve read my favorite book perhaps half-a-dozen times, I’ve seen my favorite movie  3 times and I’ve probably listened to my favorite song thousands of times.   We listen to music over and over again. We like familiar music. A music recommender has to understand the tension between familiarity and novelty.  The Netflix movie recommender will never recommend The Bourne Identity to me because it knows that I already watched it, but a good music playlist recommender had better include a good mix of my old favorites along with new music.

Highly passionate users  -There’s no more passionate fan than a music fan. This is a two-edged sword.  If your recommender introduce a music fan to new music that they like they will transfer some of their passion to your music service. This is why Pandora has such a vocal and passionate user base. On the other hand, if your recommender adds a Nickelback track to a Led Zeppelin playlist you will have to endure the wrath of the slighted fan.

Highly contextual usage – We listen to music differently in different contexts.  I may have an exercising playlist, a working playlist, a driving playlist etc.  I may make a playlist to show my friends how cool I am when I have them over for a social gathering.  Not too many people go to Amazon looking for a list of books that they can read while jogging.  A successful music recommender needs to  take context into account.

Consumed in sequences – Listening to songs in order has always been a big part of the music experience. We love playlists, mixtapes, DJ mixes, albums.  Some people make their living putting songs into interesting order.   Your collaborative filtering algorithm doesn’t have the ability to create coherent, interesting playlists with a mix of new music and old favorites

Large Personal Collections – Music fans often have extremely large personal collections – making it easier for recommendation and discovery tools to understand the detailed music taste of a listener.  A personalized movie recommender may start with a list of a dozen rated movies, while a music recommender may be able to recommend music based upon many thousands of plays, ratings skips and bans.

Highly Social – Music is social. People love to share music. They express their identity to others by the music they listen to. They give each other playlists and mixtapes.  Music is a big part of who we are.

Music is special – but of course, so are books, movies and Disney vacations – every type of item has its own special characteristics that should be taken into account when building recommendation and discovery tools. There’s no one-size-fits-all recommendation algorithm.