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Most real-world recommender services measure their performance based on the top-N results shown to the end users. Thus, advances in top-N recommendation have far-ranging consequences in practical applications. In this paper, we present a novel method, called Collaborative Denoising Auto-Encoder (CDAE), for top-N recommendation that utilizes the idea of Denoising Auto-Encoders. We demonstrate that the proposed model is a generalization of several well-known collaborative filtering models but with more flexible components.
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Collaborative Denoising Auto-Encoders
for Top-N Recommender Systems
Yao Wu Christopher DuBois Alice X. Zheng Martin Ester
Simon Fraser University Dato Inc. Dato Inc. Simon Fraser University
Burnaby, BC, Canada Seattle, WA, USA Seattle, WA, USA Burnaby, BC, Canada
ABSTRACT
Most real-world recommender services measure their performance
based on the top-N results shown to the end users. Thus, advances
in top-N recommendation have far-ranging consequences in prac-
tical applications. In this paper, we present a novel method, called
Collaborative Denoising Auto-Encoder (CDAE), for top-N recom-
mendation that utilizes the idea of Denoising Auto-Encoders. We
demonstrate that the proposed model is a generalization of several
well-known collaborative filtering models but with more flexible
components. Thorough experiments are conducted to understand
the performance of CDAE under various component settings. Fur-
thermore, experimental results on several public datasets demon-
strate that CDAE consistently outperforms state-of-the-art top-N
recommendation methods on a variety of common evaluation met-
rics.
Categories and Subject Descriptors
H.3.3 [Information Search and Retrieval]: Information Filtering
Keywords
Recommender Systems; Collaborative Filtering; Denoising Auto-
Encoders
1. INTRODUCTION
In recent years, recommender systems have become widely uti-
lized by businesses across industries. Given a set of users, items,
and observed user-item interactions, these systems can recommend
other items that the users might like. Personalized recommendation
is one of the key applications of machine learning in e-commerce
and beyond. Many recommendation systems use Collaborative Fil-
tering (CF) methods to make recommendations. In production, rec-
ommender systems are often evaluated based on the performance of
the top-N recommendations, since typically only a few recommen-
dations are shown to the user each time. Thus, top-N recommenda-
tion methods are of particular interest.
In this paper, we present a new model-based collaborative filter-
ing (CF) method for top-N recommendation called Collaborative
Permission to make digital or hard copies of all or part of this work for personal or
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republish, to post on servers or to redistribute to lists, requires prior specific permission
WSDM’16, February 22–25, 2016, San Francisco, CA, USA.
c
2015 Copyright held by the owner/author(s). Publication rights licensed to ACM.
ISBN 978-1-4503-3716-8/16/02. . . $15.00
DOI: http://dx.doi.org/10.1145/2835776.2835837
Denoising Auto-Encoder (CDAE). CDAE assumes that whatever
user-item interactions are observed are a corrupted version of the
user’s full preference set. The model learns latent representations
of corrupted user-item preferences that can best reconstruct the full
input.
1
In other words, during training, we feed the model a sub-
set of a user’s item set and train the model to recover the whole
item set; at prediction time, the model recommends new items to
the user given the existing preference set as input. Training on cor-
rupted data effectively recovers co-preference patterns. We show
that this is an effective approach for collaborative filtering.
Learning from intentionally corrupted input has been widely stud-
ied. For instance, Denoising Auto-Encoders [24] train a one-hidden-
layer neural network to reconstruct a data point from the latent rep-
resentation of its partially corrupted version. However, to our best
knowledge, no previous work has explored applying the idea to rec-
ommender systems.
CDAE generalizes several previously proposed, state-of-the-art
collaborative filtering models (see Section 3.2). But its structure
is much more flexible. For instance, it is easy to incorporate non-
linearities into the model to achieves better top-N recommendation
results. We investigate the effects of various choices for model
components and compare their performance against prior approaches
on three real world data sets. Experimental results show that CDAE
consistently outperforms state-of-the-art top-N recommendation meth-
ods by a significant margin on a number of common evaluation
metrics.
Our contributions can be summarized as follows:
• We propose a new model CDAE, which formulates the top-
N recommendation problem using the Auto-Encoder frame-
work and learns from corrupted inputs. Compared to related
methods, CDAE is novel in both model definition and objec-
tive function.
• We demonstrate that CDAE is a generalization of several
state-of-the-art methods but with a more flexible structure.
• We conduct thorough experiments studying the impact of the
choices of different components in CDAE, and show that
CDAE outperforms state-of-the-art methods on three real
world data sets.
The rest of the paper is organized as follows. Section 2 provides
the problem definition, background, and useful notations. Section 3
describes our proposed model and learning algorithm in detail. We
1
We follow the typical top-N recommendation setup and consider
only user-item interaction data in this paper. Handling of additional
data, such as user/item features and contextual information, is left
as future work.
153

discuss related work on applying neural network methods to recom-
mender systems in Section 4. Experimental results for the compo-
nents analysis and performance comparisons are presented in Sec-
tion 5. We conclude with a summary of this work and discussion
of future work in Section 6.
2. PROBLEM DEFINITION
Given a set of users U = {u = 1, ..., U }, a set of items I =
{i = 1, ..., I}, and a log of the users’ past preferences of items
O = (u, i, y
ui
), our goal is to recommend to each user u a list of
items that will maximize her/his satisfaction. In many cases, the
log contains only implicit feedback where all the y
ui
are 1; the rest
of the triples are assumed missing. We use
¯
O to denote the set of
unobserved, missing triples, and O
0
an augmented user-item pairs
dataset that includes some data sampled from
¯
O. (We discuss O
0
in more detail in the subsection on objective functions.) Let O
u
denote the set of item preferences in the training set for a particular
user u, and
¯
O
u
the unobserved preferences of user u. Items in
¯
O
u
are the candidates to be recommended to user u. The goal of the
recommender is to pick for each user u a subset of items from the
candidate set, the predicted values of which are most likely to be 1.
In some cases, y
ui
are numeric ratings in the range of, say, [1, 5]
or binary values {0, 1}. For simplicity, we only consider the case
of implicit feedback in this paper. Numeric ratings can be handled
with slight modifications to our method.
In the rest of the paper, we use u to index a user, and i and j
to index items. Vectors and matrices are denoted by bold symbols,
where symbols in lower case (e.g., x) represent vectors and sym-
bols in upper case (e.g., X) represent matrices. Unless stated dif-
ferently, x
i
also represents a vector where i is used to distinguish
different vectors. We denote the i-th row of matrix X by X
i
and
its (i, j)-th element by X
ij
.
2.1 Overview of Model-based Recommenders
Most machine learning models can be specified through two com-
ponents: model definition and objective function during training.
The model definition formulates the relationship between the in-
put (e.g., user ids, item ids, interactions, other features, etc.) and
output (ratings or implicit feedback of items). The objective func-
tion is what the training process optimizes to find the best model
parameters.
Recommender Models
In general, recommender models are defined as
ˆy
ui
= F
θ
(u, i), (1)
where ˆy
ui
is the predicted preference of user u on item i, and θ
denotes the model parameters we need to learn from training data.
Different choices of the function F
θ
correspond to different as-
sumptions about how the output depends on the input. Here we
review 4 common recommender models.
Latent Factor Model (LFM). LFM models the preference ˆy
ui
as
the dot product of latent factor vectors v
u
and v
i
, representing the
user and the item, respectively [9, 17].
2
ˆy
ui
= F
LF M
v
(u, i) = v
>
u
v
i
(2)
In addition, hierarchical latent factor models [1, 32] and the factor-
ization machine [14] can model interactions between user or item
side features.
2
To simplify notation, we assume that the latent factors are padded
with a constant to model the bias.
Similarity Model (SM). The Similarity model [8] models the user’s
preference for item i as a weighted combination of the user’s pref-
erence for item j and the item similarity between i and j. It is a
natural extension of an item-based nearest neighbor model. The
difference is that SM does not use predefined forms of item simi-
larity (e.g., Jaccard, Cosine). Instead, it learns a similarity matrix
from data [12].
ˆy
ui
= F
SM
S
(u, i) =
X
j∈O
u
\{i}
y
uj
· S
ji
(3)
Factorized Similarity Model (FSM). The problem with the Sim-
ilarity Model is that the number of parameters is quadratic in the
number of items, which is usually impractical. A straightforward
solution is to factorize the similarity matrix into two low rank ma-
trices [8, 7].
ˆy
ui
= F
F SM
p,q
(u, i) =
X
j∈O
u
\{i}
y
uj
· p
j
>
q
i
(4)
LFSM (LFM+FSM). The above models can also be combined.
For example, combining LFM and FSM results in the model SVD++
[8], which proved to be one of the best single models for the Netflix
Prize.
ˆy
ui
= F
LF SM
p,q
(u, i) =
X
j∈O
u
\{i}
y
uj
· p
j
+ p
u
>
q
i
(5)
Objective Functions for Recommenders
Objective functions for training recommender models can be roughly
grouped into two groups: point-wise and pair-wise.
3
Pair-wise ob-
jectives approximates ranking loss by considering the relative order
of the predictions for pairs of items. Point-wise objectives, on the
other hand, only depend on the accuracy of the prediction of in-
dividual preferences. Pair-wise functions are usually considered
to be more suitable for optimizing top-N recommendation perfor-
mance. However, as we demonstrate in our experiments, this is not
necessarily the case for all data sets.
Regardless of the choice of a pair-wise or point-wise objective
function, it is critical to properly take into account unobserved feed-
back within the model. Models that only consider the observed
feedback fail to account for the fact that ratings are not missing at
random. These models are not suitable for top-N recommenders
[13, 23].
Let `(·) denote a loss function and Ω(θ) a regularization term
that controls model complexity and encodes any prior information
such as sparsity, non-negativity, or graph regularization. We can
write the general forms of objective functions for recommender
training as follows.
Point-wise objective function.
X
(u,i)∈O
0
`
point
(y
ui
, ˆy
ui
) + λΩ(θ). (6)
Here O
0
denotes an augmented dataset that includes unobserved
user-item pairs. The problem with using only observed user-item
pairs is that, when users provide only implicit “like”s without ex-
plicit ratings, all the observed values y
ui
are equal to 1. In this case,
directly optimizing the point-wise objective function over O leads
3
Some models use list-wise objective functions [28, 22], but they
are not as widely adopted as point-wise and pair-wise objectives.
154
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