What’s the recipe for the ultimate road trip? Companions who can make you laugh, snacks to last the whole trip, and of course, a good music selection.
It has long been an unwritten rule that whoever’s at the wheel has control of the music. This can get boring fast, especially when the driver has a bland taste. Some passengers may tune out and start listening to their own music on headphones and disengage from the group. What is essential is a playlist that would cater to the taste of all passengers.
In this project we address the challenge of creating a playlist for multiple users with different tastes and preferences and provide a uniformly fantastic listening experience.
Imagine an app where, upon you and your companions logging in to your music devices, aggregates everyone’s listening history and automates a playlist that everyone would enjoy!
We created just an app: Orpheus. With Orpheus, multiple users can login to their Spotify accounts and find songs they can all rock out to. The order of the tracks can be based on mood, tempo and more. Orpheus was developed in Flask, using the Spotify Web API to get user data. Check it out here!
Orpheus can also be used for parties or as background music for group workouts in the gym with your bros!
In order to develop a model for recommending music, we needed data. We collected user taste profiles from the Echo Nest website. At a high level, the data was used to train a recommendation system for a single user. We employed collaborative filtering using Apache's Spark’s machine learning library to build a latent factor model. This model is then used by an aggregation strategy to determine preferences for multiple users in a group and recommend a final playlist. Finally, this playlist is sent to the Flask app where users can get groovy to it. The entire pipeline can be seen below:
The following describes each step in more detail.
The dataset the recommendation model was trained on was from the Echo Nest Taste Profile Subset. The dataset consisted of 5 GB of 1,019,318 unique users, 384,546 unique songs and 48,373,586 unique observations of user, song, play-count triplets.
On average, 125 users listen to each song and, less than 100 users are responsible for 80% of the songs listened. Most likely, there are a few songs that are highly popular and most songs are listened by a few.
With user listening history in hand, the next step was in creating a recommender system. In general, recommendation systems aim to predict the preference that a user has for a given item. Items that have the highest predicted preference can then be made as recommendations to the user.
There are two commonly used approaches to building a recommender system: Content-based filtering techniques and collaborative filtering.
In content-based filtering the system looks at the characteristics of the users or items to make predictions. For example, in music, the system would find songs in the same genre or have the same artist to determine similar songs. Using these characteristics the recommender could look at a user’s items and determine which items are most similar and recommend them.
In collaborative filtering, the idea is that users similar to you will like similar items. The content of the items is abstracted away and only the interaction between users and items is taken into account. A downside of collaborative filtering is that to make recommendations a user requires historical data with the items: This is known as the cold start problem.
Within collaborative filtering, there are two types of feedback received from users: Explicit and implicit. Explicit feedback occurs when users actively rate an item (e.g. the Netflix star rating). Implicit feedback occurs based on the consumption of an item, for example when a user listens to a song. Our dataset consists of these implicit ratings.
We chose to use the collaborative filtering approach known as the latent factor model due to its ability to handle implicit feedback, its scalability and Spark’s Alternating Least Squares (ALS) implementation. A good overview of the implementation and its practical use can be found here.
The Latent Factor Model
The latent factor model attempts to reveal latent features about users and products in order to make recommendations. Specifically, given a user-rating matrix, the model finds an approximate low-rank matrix factorization as seen below. The dot product of the user’s latent feature with an item’s latent feature represents the user’s predicted ratings. Predicted ratings for all items are computed, and ordered to give the user a final recommendation with the highest predicted rating.
The low-rank factors are determined by solving the optimization problem below.
Lambda is a regularization parameter used to avoid overfitting to the data.
The implicit model works slightly differently. Rather than attempt to predict explicit ratings, a confidence that user u likes item i is given by the following equation:
Where alpha is a tuning parameter of the model. The implicit optimization problem then becomes:
Here p_ui represents whether the user liked the item, while c_ui represents our confidence they liked the item. The regularization term is the same as in the explicit case.
For more details, see the paper that invented the implicit feedback collaborative filtering method here.
Now that we have a model, how do we choose the parameters? For that matter, how do we evaluate our model?
Ranking metrics are a common approach to evaluating recommender systems. Briefly, they allow us to assess the quality of a recommendation based on their ranking of predicted items. To evaluate our model, we used the ranking metric mean average precision (MAP). This was the ranking metric used for the million song dataset challenge on Kaggle.
Given a user’s item history and a recommendation, the precision-at-k (P) measures the proportion of correct recommendations within the top-k recommendations. The average precision (AP) is the precision at each recall point k. Finally, the mean average precision (MAP) averages the AP over all users.
In order to evaluate the model’s MAP, we perform cross-validation. Each iteration of cross-validation splits the users into training and test users. The test user’s listening history is further split to a hidden and visible set. The model is trained on both the training user’s and the test user’s visible implicit ratings. After the model is trained, the MAP score for the test user’s hidden set is computed to determine the quality of recommendations. For efficiency we selected a random subset of 100 users for computing the MAP.
To determine parameters to the model we ran 5-fold cross-validation on a one dimensional grid for each parameter: The rank, lambda and alpha. We compared the results to a baseline popularity model which simply recommends the most popular songs that a user hasn’t already listened to. The cross-validation results are given in the plot below.
We first ran the explicit model, which can be seen in Figure a). The explicit model performs considerably worse than the popularity model, which is unsurprising given that the data is not explicit and therefore this is not an appropriate model to use.
Figure b) shows the implicit model as a function of rank. Generally the higher the rank the better the performance, as higher rank matrices provide better approximations of the user-item interaction. We chose rank = 50 as a good compromise between accuracy and computational efficiency.
We found that lambda didn’t have a significant effect on the performance of the model as can be seen From Figure c). Once lambda becomes too large (> 1) the quality of the model goes down.
Conversely, alpha had a profound impact on the quality of the model. We found the optimal value of alpha = 40, which is also the suggested values by the authors of the algorithm.
We found the optimal parameters to be rank = 50, lambda = 0.1 giving a final MAP score of 0.1.
When comparing this to the closed Kaggle competition, we get approximately 25th out of 150 teams. This gave us confidence the model was performing well.
We now have a working recommender system and it works great for individual recommendations. The next part is to make Orpheus recommend for a group of users. But how do we convert a single recommender system into a group recommender system?
One technique is to get each group member’s recommendation and combine all the recommendations using an aggregation strategy.
As much as possible, we want Orpheus to come up with a playlist which would satisfy all members of the group. Assuming there are a couple of different aggregation strategies to employ, which one would work best for a small group? What if in our road trip scenario we had a minivan instead of a car, would the same aggregation strategy work on a larger group? More importantly, how would Orpheus recommend to a group with very dissimilar tastes?
These were the questions we needed to answer to come up with the best recommendation for the group.
To illustrate how aggregation strategies work, we pick some users in our dataset. Suppose User 193650 and his friends, User 84250, and User 92650 go for a drive, Orpheus knows their listening history and has come up with individual recommendations for each of them. Table 1 shows a subset of recommended songs and how confident Orpheus is that the user will like the song. A confidence closer to 1.0 means the user will most likely enjoy the song. Which songs will Orpheus play first?
The least misery aggregation strategy has been used for a group recommendation system for movies. The movie recommender uses it on explicit ratings while here we use it on implicit ratings. For each song, we get the smallest confidence rating and set it as the confidence rating of the song for the group. We then rearrange all songs from highest to lowest confidence rating. This is now the group recommended playlist.
Basically, least misery gets the happiness of the least happy member of the group. The other strategies we tested are: Average strategy, which gets the average happiness of all members; most pleasure, which considers the happiness of the most happy member; and multiplicative, which gets the product of all members’ happiness.
To measure the satisfaction of each member of a playlist, we apply the formula below taken from here, which in turn can give us the group’s satisfaction rating on the resulting playlist.
Another variable we need to consider is homogeneity of the group. K-means clustering on the dataset would uncover similarities of each users and group them together based on their taste. This way, we can just get members from the same cluster for a homogenous group and members from different clusters for a heterogenous group.
However, the main challenge with the dataset is its dimensionality and sparsity. Imagine doing K-means clustering for 1 million observations and close to 400,000 variables! Luckily, in the process of training a recommender system, a reduced latent factor matrix is produced. We apply K-means clustering on the user’s latent features to come up with groups.
To test our aggregation strategies, we divided our groups into two categories: homogenous grouping and heterogeneous grouping. For each category we wanted to see how varying the group size might affect the group satisfaction so we made 3, 5, and 7 member groups. We made 20 samples for each group for a total of 120 samples.
The statistical results below show that the group satisfaction of homogenous groups using different aggregation strategies do not statistically differ from each other. This means that any of the aggregation strategies will result to a playlist where all members are happy in the homogeneous case. For heterogeneous groups, we found that the average strategy was statistically significantly better than other methods, as can be seen by the ANOVA and Tukey HSD post-hoc test results below.
The Flask App
We complete our project with a Flask Application designed to generate a Spotify playlist ordered in whichever feature chosen from the tracks of up to six different people. The diagram below shows how the app works.
After the necessary inputs are made, the playlist can be launched with a player embedded inside the application. Users can order the playlist by metrics such as energy, mood and tempo, as can be seen in the pictures below.
Orpheus gives people the opportunity to enjoy music together. A flask app supported by an implicit collaborative filtering recommender system combined with appropriate aggregation strategies give users the ultimate tool to accompany them on the road, at the zoo, or in the bedroom.
We encourage you to grab your friends and experience Orpheus today!
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