my-movie-recommender

This repository contains a variety of recommender system work. Some of the work pertains to making user-movie recommendations through a couple different matrix factorization methods - the project description contained in this readme is specific to that. The newer work pertains to using deep learning to develop a hybrid item-to-item content-collaborative recommender - this work is described in great detail in this blog post.

Dev Setup

• cd into repository root
• conda env create -f environment.yml
• source activate my-movie-recommender
• ipython kernel install --user --name=my-movie-recommender

Ensure PyTorch is installed

conda install pytorch torchvision -c pytorch (works for Linux and MacOS)

Introduction

I like to watch movies but it's becoming increasingly difficult to find new movie choices. I'd rather not rewatch and I haven't had much luck manually scouring the web for random movie ranking lists. So, I started this project to develop a movie recommender system to discover new movies to watch that I will [hopefully] enjoy watching.

I will be using the MovieLens 20M Dataset and various recommender techniques. Below is a dataset description from grouplens:

Stable benchmark dataset. 20 million ratings and 465,000 tag applications applied to 27,000 movies by 138,000 users. Includes tag genome data with 12 million relevance scores across 1,100 tags. Released 4/2015; updated 10/2016 to update links.csv and add tag genome data.

The first technique I'm using is Probabilistic Matrix Factorization (PMF). I've written the core algorithm from scratch myself, so this will be a good opportunity to see how well my code performs on a large dataset and generally how well the PMF method performs. My current plan is to perform all computations locally; if this becomes infeasible I will look into cloud computing options.

The general structure of this project is to store classes and other source code in the src directory and IPython Notebooks in the notebooks directory. The presentation of the work will be in notebook form.

Developing My Ratings Profile

It didn't take long to realize that manually rating over 27,000 movies was not feasible. To adjust for this, I decided to filter the list of movies for those I was more likely to have seen. I developed several different filters, each one bringing my task of creating a ratings profile closer to being reasonable:

1. Title length. I noticed a lot of foreign movies tend to have longer titles. A lot of times they have a translation of the title appended to the original title. So I decided to filter out movies with titles greater than 60 characters. (I chose 60 because 'The Lord of the Rings: The Fellowship of the Ring' is 49 characters and I wouldn't want to miss out on that!). This brought the list of movies down from 27278 to 26184. Not that great, but a start.
2. English words. If the title of a movie isn't in English, there's a good chance I've never seen it. So I downloaded a large list of English words and excluded movies with titles having more than 2 words not in that list. This brought the list down from 26184 to 22501. Not bad.
3. Release year. I was born in the late 80's, and have never been a big fan of older movies. Maybe I'm missing out, but regardless, if the movie was released before 1990, I probably haven't seen it. List dropped from 22501 to 13882. Now that's what I'm talking about. But wait a second... 13,000 is still way too many. I don't want to spend my weekends punching in numbers from 1-5.
4. Rating frequency. The movie rating frequency in the 20M dataset has a mean of 747, a standard deviation of 3085, a max of 67,310, a median of 18 and is heavily right skewed. I chose my cutoff to be 200, which brought my list down from 13882 to 4055. 4000 sounded doable, so I did it.

Of the 4055 movies I was prompted with, I ended up rating 612 of them. Here is the distribution of my ratings:

5 stars - 260
4 stars - 215
3 stars - 107
2 stars - 24
1 star - 6

I guess I tend to give movies high ratings. I wonder what the population distribution is on movie ratings and if it follows the same trend. There are probably some interesting sociology/psychology studies on this.

Regardless of how my distribution stacks up with everyone else, I don't expect it to affect the results of the recommender.

Probabilistic Matrix Factorization Model (PMF)

Model Selection (hyperparameter tuning)

One of the more challenging tasks in this project is figuring out what model to use. There are three primary hyperparameters that must be selected for a particular PMF model:

1. Rank

This is the rank of the matrix factorization. The sparse matrix for this dataset has a size ~ 140,00 x 27,000. The idea is that you can factorize this massive and very sparse matrix into the multiplication of two much smaller [and more manageable] matrices. The rank of the factorization essentially determines the size of the two smaller matrices.

PMF is learning a projection of both movies and users into the same latent space. Originally users exist in a 27,000 dimensional space, and movies exist in a 140,000 dimensional space. After PMF is performed, movies and users will exist together in a space with the number of dimensions determined by the rank. This is a bit of foreshadowing, but once users and movies exist in the same space you can easily see how movie recommendations can be based on how "nearby" or "close" movies are to users in that space.

The new dimensions in this latent space can be thought of as "concepts". For example, there could be a "sci-fi" or a "drama" concept. Based on the interplay between users and movies in the ratings data, you can map them both to these common concepts or dimensions. When choosing the rank to use, one must realize they are limiting the number of concepts that can be learned by the machine.

Based on recommendations from others, my own intuition, and practical concerns for computation time, I have chosen to use rank 10.

2. Lambda.

PMF assumes that the rows in the decomposed matrices (which are vectors embedding users and movies into latent space) come from a Gaussian distribution with mean 0 and a covariance matrix equal to lambda^(-1) * I. Lambda is a regularization parameter that controls the variance of the prior distributions on the rows of these low rank factorized matrices U and V. Based on the results of a grid search performed in notebooks/5-model-selection-PMF.ipynb, a value of 7.5 is selected for lambda.

3. Variance.

PMF assumes a 1D Gaussian distribution on the individual elements of the sparse ratings matrix. Variance here refers to the variance of this distribution. As a result of the grid search, a value of 0.5 is selected for variance.

Singular Value Decomposition Model (SVD)

An SVD model was also applied in this project. SVD factorizes the sparse ratings matrix into the product of three low rank matrices: the left-singular vectors, right-singular vectors, and the non-zero singular values. Similar to SVD, the rows of U and V, when multiplied by the singular values matrix, represent an embedding of users and movies into a shared latent space. Once the low rank decomposition has been found, predictions are made by computing the matrix multiplication, or a subset thereof.

For SVD I was able to take advantage of scipy's compressed sparse column (CSC) matrix and an SVD algorithm optimized for operating on CSC matrices. This helped SVD significantly outperform PMF in computation time.

Copyright Stuff

Copyright 2018 Adam Lineberry

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
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    http://www.apache.org/licenses/LICENSE-2.0

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