# PyMC3 Implementation of Probabilistic Matrix Factorization (PMF): MAP produces all 0s

I've started working with pymc3 over the past few days, and after getting a feel for the basics, I've tried implementing the Probabilistic Matrix Factorization model.

For validation, I use a subset of the Jester dataset. I take the first 100 users who rated all 100 jokes. I use the first 20 jokes and leave the ratings unchanged; they are in the range [-10, 10] for all jokes. For ease of reference, I've made this subset available here.

import pymc3 as pm
import numpy as np
import pandas as pd
import theano

n, m = data.shape
test_size = m / 10
train_size = m - test_size

train = data.copy()
train.ix[:,train_size:] = np.nan  # remove test set data
train[train.isnull()] = train.mean().mean()  # mean value imputation
train = train.values

test = data.copy()
test.ix[:,:train_size] = np.nan  # remove train set data
test = test.values

# Low precision reflects uncertainty; prevents overfitting
alpha_u = alpha_v = 1/np.var(train)
alpha = np.ones((n,m)) * 2  # fixed precision for likelihood function
dim = 10  # dimensionality

# Specify the model.
with pm.Model() as pmf:
pmf_U = pm.MvNormal('U', mu=0, tau=alpha_u * np.eye(dim),
shape=(n, dim))
pmf_V = pm.MvNormal('V', mu=0, tau=alpha_v * np.eye(dim),
shape=(m, dim))
pmf_R = pm.Normal('R', mu=theano.tensor.dot(pmf_U, pmf_V.T),
tau=alpha, observed=train)

# Find mode of posterior using optimization
start = pm.find_MAP()  # Find starting values by optimization


This all appears to work splendidly, but the values produced by find_MAP end up being all 0s for both U and V, as can be seen by running:

(start['U'] == 0).all()
(start['V'] == 0).all()


I am relatively new to both Bayesian modeling and pymc, so I could easily be missing something obvious here. Why am I getting all 0s?

I did two things to fix your code. One was to initialize the model away from zero, the other one was to use a non-gradient based optimizer:

import pymc3 as pm
import numpy as np
import pandas as pd
import theano
import scipy as sp

n, m = data.shape
test_size = m / 10
train_size = m - test_size

train = data.copy()
train.ix[:,train_size:] = np.nan  # remove test set data
train[train.isnull()] = train.mean().mean()  # mean value imputation
train = train.values

test = data.copy()
test.ix[:,:train_size] = np.nan  # remove train set data
test = test.values

# Low precision reflects uncertainty; prevents overfitting
alpha_u = alpha_v = 1/np.var(train)
alpha = np.ones((n,m)) * 2  # fixed precision for likelihood function
dim = 10  # dimensionality

# Specify the model.
with pm.Model() as pmf:
pmf_U = pm.MvNormal('U', mu=0, tau=alpha_u * np.eye(dim),
shape=(n, dim), testval=np.random.randn(n, dim)*.01)
pmf_V = pm.MvNormal('V', mu=0, tau=alpha_v * np.eye(dim),
shape=(m, dim), testval=np.random.randn(m, dim)*.01)
pmf_R = pm.Normal('R', mu=theano.tensor.dot(pmf_U, pmf_V.T),
tau=alpha, observed=train)

# Find mode of posterior using optimization
start = pm.find_MAP(fmin=sp.optimize.fmin_powell)  # Find starting values by optimization

step = pm.NUTS(scaling=start)
trace = pm.sample(500, step, start=start)


This is an interesting model that would make a great contribution. Please consider adding this, once you're certain it works as desired, to the examples folder and do a pull request.

• Thanks for the fixes! Do you mind commenting on why these changes are necessary to avoid the 0-valued start matrices? – Mack Apr 16 '15 at 16:49
• Does it produce sensible results that way? Unfortunately I don't have a good explanation. I kind of intuited that starting at 0 would be a very odd space in the likelihood with an ill-defined gradient (I'm not sure if that's actually the case). This still didn't work with the gradient based optimization so I tried Powell which usually is pretty robust. You could try still starting at 0 with Powell which might be good enough. – twiecki Apr 17 '15 at 13:21
• Just going off of RMSE, it seems the MAP estimate is reasonable. The test RMSE for 5.0823, which is slightly better than what comes out from using a simple baseline. If I just replace all missing values by the global mean, the test RMSE is 5.0839. The best I can produce with libFM is 4.980. – Mack Apr 17 '15 at 17:44
• Note that the 4.980 number mentioned above was incorporating bias terms, both global and per-item/per-joke. That model was also using 8 dimensions. If I leave out the bias terms and use 10 dimensions, as I do for the PMF model, then the test RMSE is around 5.2 using libFM. This model is similar to a regularized SVD, which PMF is supposed to improve upon. – Mack Apr 17 '15 at 18:27
• Following the advice of Salakhutdinov and Mnih, p.886, I also monitored the Frobenius norms of the sampled U and V matrices. They converge after 7-10 samples, which I believe is quite rapid. This also leads me to believe the MAP estimate is reasonable. Interestingly, the MAP estimate actually produces a better RMSE than the sampler, which gives a test RMSE of 5.108 using 400 samples, after discarding 10 samples as burn-in. – Mack Apr 17 '15 at 18:37