Unsupervised Cluster Matching via Probabilistic Latent Variable Models
Tomoharu Iwata and Tsutomu Hirao and Naonori Ueda
NTT Communication Science Laboratories
2-4 Hikaridai, Seika-cho, Soraku-gun, Kyoto, Japan

Abstract
We propose a probabilistic latent variable model for unsupervised cluster matching, which is the task of finding
correspondences between clusters of objects in different
domains. Existing object matching methods find oneto-one matching. The proposed model finds many-tomany matching, and can handle multiple domains withdifferent numbers of objects. The proposed model assumes that there are an infinite number of latent vectors that are shared by all domains, and that each object is generated using one of the latent vectors and adomain-specific linear projection. By inferring a latentvector to be used for generating each object, objects indifferent domains are clustered in shared groups, andthus we can find matching between clusters in an unsupervised manner. We present efficient inference procedures for the proposed model based on a stochastic EM
algorithm. The effectiveness of the proposed model isdemonstrated with experiments using synthetic and real
data sets.

1

Introduction

Object matching is an important task for finding correspondences between objects in different domains. Examples ofobject matching include matching an image with an annotation (Socher and Fei-Fei 2010), an English word with aFrench word (Tripathi, Klami, and Virpioja 2010), and auser identification with a user identification in a different
database for recommendation (Li, Yang, and Xue 2009).
Most object matching methods require similarity measures
between objects in different domains, or paired data that
contain correspondence information.
Similarity measures and correspondences might not be
available. Defining similarities and generating correspondences incur a cost and require time, and they are sometimes
unobtainable because of the need to preserve privacy. For
this situation, unsupervised object matching methods havebeen proposed, such as kernelized sorting (Quadrianto et al.
2010), least squares object matching (Yamada and Sugiyama
2011), matching canonical correlation analysis (Haghighi etal. 2008), and variational Bayesian matching (Klami 2012).
Copyright c 2013, Association for the Advancement of Artificial
Intelligence (www.aaai.org). All rights reserved.

These methods find one-to-one matching. However, matching is not necessarily one-to-one in some applications. For
example, in image annotation, an annotation could be attached to multiple images that show the same thing, and an
image that shows multiple things could have multiple annotations. When matching English and French vocabularies,
multiple English words with the same meaning could correspond to multiple French words with the same meaning.
Other limitations of these methods are that the number of domains needs to be two, and the numbers of objects in different domains must be the same. There can be more than two
domains in some applications, for example matching multilingual vocabularies such as English, French and German,
and the vocabulary size of the languages is usually different.
In this paper, we propose a probabilistic latent variable
model for unsupervised cluster matching, which is a taskthat involves the many-to-many matching of objects in multiple domains. The proposed model can handle more than
two domains with different numbers of objects. The proposed model is called the many-to-many matching latentvariable model (MMLVM), and assumes that there are an infinite number of latent vectors that are shared by all domains.
Each object is generated using one of the latent vectors anda domain-specific linear projection. The latent vector to be
used for generating an object is unknown. By assigning a latent vector for each object, we can cluster objects in different
domains, and find matching between clusters. The numberof clusters is automatically inferred from the given data byusing a Dirichlet process prior. Because the proposed model
is a probabilistic generative model, we can extend it in aprobabilistically principled manner, and use it, for example,
for handling missing data, integration with other probabilistic models, and generalization to exponential family distributions. The inference of the proposed model is based ona stochastic EM algorithm, in which the Gibbs sampling ofcluster assignments and the maximum joint likelihood estimation of projection matrices are alternately iterated whilemarginalizing out latent vectors.
The remainder of this paper is organized as follows. We
formulate the proposed model in Section 2, and describe efficient inference procedures for the proposed model in Section 3. In Section 4, we briefly review related work. In Section 5, we demonstrate the effectiveness of the proposedmodel with experiments using synthetic and real data sets.

Symbol
D
Nd
Md
K

latent vectors

Table 1: Notation.
Description
number of domains
number of objects in the dth domain
dimensionality of the dth domain
dimensionality of a latent vector

Z

W2

W1

Finally, we present concluding remarks and a discussion of
future work in Section 6.

2 Proposed model
Suppose that we are given objects in D domains X =Nd{Xd }D
d=1 , where Xd = {xdn }n=1 is a set of objects in theMd
dth domain, and xdn ∈ R
is the feature vector of thenth object in the dth domain. Our notation is summarized inTable 1. Note that we are unaware of any correspondences
between objects in different domains. The number of objects Nd and the dimensionality Md for each domain can bedifferent from those of other domains. The task is to matchclusters of objects across multiple domains in an unsupervised manner.
The model proposed for this task is a probabilistic latentvariable model. The proposed model assumes that there arepotentially a countably infinite number of clusters, and eachcluster j has a latent vector zj ∈ RK in a K-dimensional
latent space. Each object xdn in the dth domain is generated
depending on a domain-specific projection matrix Wd ∈RMd ×K and a latent vector zsdn that is selected from a setof latent vectors Z = {zj }∞j=1 . Here, sdn ∈ {1, · · · , ∞}is the latent cluster assignment of object xdn . Objects that
use the same latent vector, or that have the same cluster assignment sdn , are considered to match. Figure 1 shows therelationship between latent vectors and objects in two domains, where arrows that indicate the corresponding latentvectors for each object are hidden.
Specifically, the proposed model is an infinite mixture
model, where the probability of object xdn is given byp(xdn |Z, W , θ) =∞∑θj N (xdn |Wd zj , α−1 I),

(1)

j=1

where W = {Wd }D
d=1 is a set of projection matrices,θ = (θj )∞j=1 is mixture weights, θj represents the probability that the jth cluster is chosen, and N (µ, Σ) denotes thenormal distribution with mean µ and covariance matrix Σ.
In the proposed model, a set of latent vectors Z are shared
among multiple domains, but projection matrix Wd depends
on the domain. By sharing the latent vectors, we can assign
objects in different domains to common clusters, and findmatching between clusters. By employing domain-specificprojection matrices, we can handle multiple domains withdifferent dimensionalities and different properties. Given latent vectors, an arbitrary number of objects can be generated
for each domain independently. Therefore, we can handle
domains with different numbers of objects.
In summary, the proposed model generates objects in multiple domains X according to the following process,

X1

X2

objectsin domain 1

objectsin domain 2

Figure 1: Relationship between latent vectors and objects in
two domains.

γ

a
b

α

z

θ

s

x

∞
N

r
W
D

Figure 2: Graphical model representations of the proposedmodel.
1. Draw cluster proportions
θ ∼ GEM(γ)
2. Draw a precision parameterα ∼ Gamma(a, b)
3. For each cluster: j = 1, . . . , ∞(a) Draw a latent vectorzj ∼ N (0, (αr)−1 I)
4. For each domain: d = 1, . . . , D(a) For each object: n = 1, . . . , Nd
i. Draw a cluster assignment
sdn ∼ Discrete(θ)
ii. Draw an observation vector
xdn ∼ N (Wd zsdn , α−1 I)
Here, GEM(γ) is the stick-breaking process (Sethuraman1994) that generates mixture weights for a Dirichlet processwith concentration parameter γ. By using a Dirichlet process, we can automatically find the number of clusters fromthe given data. Figure 2 shows graphical model representations of the proposed model.
The joint probability of the data X and the cluster assignDdments S = {{sdn }Nn=1 }d=1 is given byp(X, S|W , a, b, r, γ) = p(S|γ)p(X|S, W , a, b, r). (2)By marginalizing out mixture weights θ, the first factor is
calculated by∏Jγ J j=1 (N·j − 1)!
p(S|γ) =,(3)γ(γ + 1) · · · (γ + N − 1)∑D
where N =d=1 Nd is the total number of objects, N·jrepresents the number of objects assigned to cluster j, and J
is the number of clusters for which N·j > 0. By marginalizing out latent vectors Z and precision parameter α, thesecond factor of (2) is calculated byp(X|S, W , a, b, r)= (2π)−

P

d Md Nd
2

KJ

r 2

Here,
a0 = a +b0 = b +J
1
ba Γ(a0 ) ∏|Cj | 2 .
0b0a Γ(a) j=1

p(X|sdn = j, S\dn , W , a, b, r)p(X\dn |S\dn , W , a, b, r)(4)

Cj−1 =

D
∑

Wd>

M
− 2d

\dn
1
b\dn
Γ(a0sdn =j ) |Cj,sdn =j | 2,
1
0a0s =j Γ(a0
|Cj\dn | 2
\dn )
b dn

(13)

sdn =jd=1 Md Nd
,
2

(5)

∑

where subscript sdn = j indicates the value when object
xdn is assigned to cluster j as follows,a0sdn =j = a0 ,

(6)

xdn ,

(7)

Ndj Wd> Wd + rI,

(8)

d=1

0a0

= (2π)

d=1µj = Cjusing (3). By using (4), the second factor is given by

∑D

D Nd
J
1 ∑∑
1 ∑ > −1x>
µ C µj ,dn xdn −2
2 j=1 j jn=1
D∑

where \dn represents a value or set excluding the nthobject in the dth domain, and we use the fact thatp(sdn = j|X, S\dn , W , a, b, r, γ) does not depend onp(xdn |W , a, b, r). The first factor is given by{
N·j\dnp(sdn = j, S\dn |γ)
for existing cluster
N −1+γ=γ
for new cluster,p(S\dn |γ)N −1+γ(12)

n:sdn =j(14)

1
1b0sdn =j = b0\dn + x>
xdn + µ>C −1 µj\dn
2 dn
2 j\dn j\dn
1
C −1
− µ>
µj,sdn =j ,(15)
2 j,sdn =j j,sdn =j−1µj,sdn =j = Cj,sdn =j (Wd> xdn + Cj\dnµj\dn ),

(16)

d=1

where Ndj is the number of objects assigned to cluster j
in domain d. The posterior for the precision parameter α isgiven byp(α|X, S, W , a, b) = Gamma(a0 , b0 ),(9)and the posterior for the latent vector zj is given byp(zj |X, S, W , r) = N (µj , α−1 Cj ).

(10)

3 Inference
We describe the inference procedures for the proposedmodel based on a stochastic EM algorithm, in which collapsed Gibbs sampling of cluster assignments S and the
maximum joint likelihood estimation of projection matrices
W are alternately iterated while marginalizing out the latentvectors Z and the precision parameter α.
In the E-step, given the current state of all but one latentassignment sdn , a new value for sdn is sampled from thefollowing probability,p(sdn = j|X, S\dn , W , a, b, r, γ)=∝p(sdn = j, S\dn |γ)p(X|sdn = j, S\dn , W , a, b, r)p(S\dn |γ)p(X\dn |S\dn , W , a, b, r)p(xdn |W , a, b, r)p(sdn = j, S\dn |γ) p(X|sdn = j, S\dn , W , a, b, r)·,p(S\dn |γ)p(X\dn |S\dn , W , a, b, r)
(11)

−1
−1
Cj,s
= Wd> Wd + Cj\dn
.
dn =j

(17)

In the M-step, the projection matrices W are estimatedby maximizing the logarithm of the joint likelihood (2). We
can maximize it by using a gradient-based numerical optimization method such as the quasi-Newton method (Nocedal 1980). The gradient of the joint log likelihood is calculated byJ∑∂ log p(X, S|W , a, b, r, γ)= −Wd
Ndj Cj
∂Wd
j=1

−

J
)
∑
a0 ∑(
>
>
N
W
µ
µ
−
x
µ
.
dj
d
j
dn
j
j
b0 j=1n:s =j(18)dn

By iterating the E-step that samples the cluster assignment
sdn by employing (11) for all objects n = 1, . . . , Nd in eachdomain d = 1, . . . , D, and the M-step that maximizes thejoint likelihood using (18) with respect to the projection matrix Wd for all domains d = 1, . . . , D, we can obtain an
estimate of the cluster assignments and projection matrices.
We can use cross-validation to select an appropriate dimensionality for the latent space K. With cross-validation,
we assume that some features are missing in the given data,and infer the model with different K. Then, we select the Kvalue that performed the best at predicting missing values.

Missing data
The proposed model can handle missing data. Let hdn =d(hdnm )M
m=1 be a vector indicating observed indexes, where
hdnm = 1 if xdnm is observed, hdnm = 0 otherwise, and
Mdn is the number of observed values for object xdn . The
posterior parameters are calculated as follows,∑D ∑NdMdn0a = a + d=1 n=1
,
(19)
2
d
1 ∑∑
(hdn ◦ xdn )> (hdn ◦ xdn )
2
n=1

N

D

b0 = b +

d=1

−

J
1∑

2 j=1

−1
µ>
j Cj µj ,µj = Cj

Cj−1 =

D
∑
d=1

D
∑

Wd>

(20)
∑

hdn ◦ xdn ,

d=1

n:sdn =j( ∑)diag(hdn ) Wd + rI,

Wd>

(21)

(22)

n:sdn =jwhere ◦ represents the Hadamard product, or element-wiseproduct, and diag(hdn ) returns a diagonal matrix whose diagonal elements are hdn1 , . . . , hdnMd .

4 Related work
Unsupervised object matching is a task that involves finding correspondences between objects in different domains
without correspondence information. For example, kernelized sorting (Quadrianto et al. 2010) finds correspondencesby permuting a set to maximize the dependence between two
domains. Here, the Hilbert Schmidt Independence Criterion(HSIC) is used for measuring dependence. Kernelized sorting requires the numbers of objects in different domains tobe the same. Kernelized sorting is extended to convex kernelized sorting (Djuric, Grbovic, and Vucetic 2012), whichis guaranteed to find a globally optimal solution. Matching canonical correlation analysis (MCCA) (Haghighi etal. 2008) is another unsupervised object matching method,where bilingual translation lexicons are learned from twomonolingual corpora. MCCA simultaneously finds latentvariables that represent correspondences and latent vectors
so that the latent vectors of corresponding objects exhibit the
maximum correlation. (Tripathi et al. 2011) also proposeda method for unsupervised object matching that is related
to MCCA. These methods assume the one-to-one matchingof objects in two domains. On the other hand, the proposedmodel can find many-to-many matching, and is applicableto objects in more than two domains.
Manifold alignment is related to the proposed model because they both find latent vectors of multiple sets in a joint
latent space. The unsupervised manifold alignment method(Wang and Mahadevan 2009) finds latent vectors of different domains in a joint latent space in an unsupervised manner. The method first identifies all the possible matches foreach example leveraging its local geometry, and then finds
an embedding in the latent space. The method requires permutations of the factorial of the number of neighborhoods tomatch the local geometry.
There has been some work on improving the learning performance of a classification task by using labeled objects
in different domains without correspondence information.
For example, multiple outlook mapping (MOMAP) (Hareland Mannor 2011) improves the performance by matching
the moments of the empirical distributions for each class of
two domains. (Shi et al. 2010) proposed a transfer learning
method that improves the learning performance by embedding both source and target domains in a joint latent spacewhen a limited number of target objects are labeled. Thesemethods require labeled objects. On the other hand, the proposed method does not require any labeled objects.
The proposed model is an extension of probabilistic
canonical correlation analysis (CCA) (Bach and Jordan
2005), which finds dependences between objects in two domains by projecting objects in a latent space. Probabilistic
CCA requires correspondence information between different domains since it takes a set of paired objects as input. On
the other hand, the proposed model can find dependences inan unsupervised manner without correspondence information by taking a set of objects for each domain as input. CCA
is successfully used for a wide variety of applications, suchas multi-label prediction (Rai and Daumé III 2009; Sun, Ji,
and Ye 2011), information retrieval (Hardoon, Szedmak, andShawe-Taylor 2004), and image annotation (Kimura et al.
2010). The proposed model can be used for these applications when supervised data are unavailable.
The proposed model can be seen as a generalization of theinfinite Gaussian mixture model (Rasmussen 2000). Whenthe dimensionality of the latent space is the same as that of
the observed space D = K and Wd = I for all domains, theproposed model corresponds to the infinite Gaussian mixture
model. The proposed model is different from the mixture of
PCA (Tipping and Bishop 1999), where latent vectors are
not shared among different objects.

5

Experiments

We evaluated the proposed model quantitatively by using
three synthetic and four real data sets. The statistics of theseven data sets are shown in Table 2. There are two domains
for all the data sets. Synth3, Synth5 and Synth10 are synthetic data sets with different true dimensionalities of thelatent space K ∗ = 3, 5 and 10, respectively. We generated
the synthetic data sets using the following procedure. First,
we sampled latent vectors zj for j = 1, · · · , J ∗ from a K ∗ dimensional normal distribution with mean 0 and covariance
I. Then, we generated projection matrices Wd for d = 1, 2,
where each element is drawn from a normal distribution withmean 0 and variance 1. Finally, we generated N/J ∗ objects
for each cluster j using a normal distribution with mean
Wd zj and covariance α−1 I, and obtained N objects in total for each domain d = 1, 2. Iris, Glass, Wine and MNIST
are the real data sets, which were obtained from LIBSVM
multi-class data sets (Chang and Lin 2011), and generated
objects in two domains by randomly splitting the features

1

0.8
adjusted Rand index

Table 2: Statistics of the data sets: the number of objects N ,the dimensionality of the objects Md , and the true number ofclusters J ∗ , and the true dimensionality of the latent space
K ∗.
N1 /N2 M1 M2 J ∗K∗Synth3
200
50
50
5
3
Synth5
200
50
50
5
5
Synth10
200
50
50
5
10
Iris
150
2
2
3 N/A
Glass
214
4
5
7 N/A
Wine
178
6
7
3 N/A
MNIST
200 392 392 10 N/A

0.6

0.4

0
1

into two parts for each data set as (Quadrianto et al. 2010;
Djuric, Grbovic, and Vucetic 2012) did for their experiments. Because there is no overlapping feature, we cannot
calculate similarities between objects in different domains.
For the evaluation measurement, we used the adjusted
Rand index (Hubert and Arabie 1985), which quantifies thesimilarity between inferred clusters and true clusters, andtakes the value from −1 to 1, and gives 0 for random clustering. For the real data sets, we assume that the category labelof each object is its true cluster assignment. With unsupervised cluster matching, the adjusted Rand index measures
how well objects with the same label in different domains
are assigned to the same cluster as well as measuring theclustering performance within each domain.
With the proposed method, we used the dimensionality of the latent space K = 5, and set the hyperparameters a = 1, b = 1, r = 1, γ = 1 for all the data sets.
To alleviate the local optimum problem, we ran the inference five times with different initial conditions, and selected
the result that achieved the highest likelihood. For comparison, we used k-means (KM), convex kernelized sorting (CKS) (Djuric, Grbovic, and Vucetic 2012), and their
combinations (KM-CKS and CKS-KM) as described below.
The KM method is widely used for clustering. Although
KM is not a cluster matching method, we included KM as
a baseline method to show the adjusted Rand index when
only objects in the same domain were clustered. The CKS
method is an unsupervised object matching method. It directly finds correspondence between objects, and does notcluster objects in the same domain. With KM-CKS, first
we discovered clusters by using KM for each domain individually, and then found the correspondence between clusters in two domains by using CKS. We used the mean
vector of each cluster as the input for matching clusters
by CKS. With the CKS-KM, after matching objects using CKS, we combined matched objects in two domains
into a vector, and estimated clusters using KM. We employed CKS as comparison methods since it achieved higher
performance than kernelized sorting and matching canonical correlation analysis (Djuric, Grbovic, and Vucetic 2012;
Jagarlamudi, Juarez, and Daumé III 2010). With KM, KMCKS and CKS-KM, we used the number of clusters estimated by the proposed model. For comparison with object
matching based methods (CKS, CKS-KM), we used data

Proposed
KM−CKSCKS−KM
KM
CKS

0.2

2

3

4
5
6
7
latent dimensionality: K

8

9

10

Figure 3: Adjusted Rand index achieved by the proposedmodel with different latent dimensionalities K using theSynth5 data set whose true latent dimensionality is K ∗ = 5.

sets that had the same numbers of objects in the two domains
N1 = N2 for each data set.
Table 3 shows the adjusted Rand index for the seven data
sets, which were averaged over 10 experiments for each data
set. For all the data sets, the proposed model achieved thehighest adjusted Rand index. This result indicates that the
proposed model can infer matching clusters by assuming ashared latent space. KM-CKS achieved higher performance
than KM by matching clusters as a post-processing step.
With KM-CKS, since clusters are inferred individually foreach domain, the estimated clusters might be different indifferent domains. On the other hand, since the proposedmodel infers clusters simultaneously for all domains, it successfully found shared clusters compared with KM-CKS as
shown by the higher adjusted Rand index than KM-CKS.
The adjusted Rand index obtained with the CKS method
was low, because it does not cluster objects. By clustering
the result of CKS, CKS-KM improved the cluster matching
performance. However, it did not outperform the proposedmodel, because errors accumulated in object matching by
CKS cannot be corrected in the clustering process with kmeans.
Figure 3 shows the adjusted Rand index achieved by the
proposed model with different latent dimensionalities using
the Synth5 data set. The value was highest when the latentdimensionality of the model was the same as the true latentdimensionality K = K ∗ = 5. The proposed model with
K 6= K ∗ also performed better than the other methods. This
result indicates that the proposed model is robust to the latentdimensionality setting because of the Bayesian inference.
Figure 4 shows the adjusted Rand index with different
numbers of domains D using the Synth5 data set. With KMCKS, CKS-KM and CKS, we found matching across multiple domains by matching clusters/objects between the Dthdomain and each of the other D − 1 domains, and then combined the results. In general, the adjusted Rand index decreases as the number of domains increases, since the number of possible combinations of cluster matching increases.

Table 3: Average adjusted Rand index and its standard deviation. Values in bold typeface are statistically better from those innormal typeface as indicated by a paired t-test.
Proposed
KM
KM-CKSCKSCKS-KM
Synth30.875 ± 0.101 0.525 ± 0.014 0.589 ± 0.117 0.014 ± 0.005 0.699 ± 0.135
Synth50.893 ± 0.126 0.548 ± 0.029 0.583 ± 0.198 0.006 ± 0.007 0.571 ± 0.182
Synth10 0.827 ± 0.145 0.556 ± 0.026 0.553 ± 0.165 0.009 ± 0.006 0.678 ± 0.170
Iris
0.383 ± 0.189 0.224 ± 0.091 0.254 ± 0.154 0.003 ± 0.002 0.207 ± 0.089
Glass
0.160 ± 0.020 0.050 ± 0.008 0.052 ± 0.011 0.001 ± 0.001 0.047 ± 0.010
Wine0.222 ± 0.111 0.125 ± 0.025 0.142 ± 0.046 0.001 ± 0.001 0.107 ± 0.038
MNIST 0.085 ± 0.016 0.030 ± 0.007 0.037 ± 0.008 0.008 ± 0.005 0.041 ± 0.016
1

adjusted Rand index

0.8

0.6

0.4

0.2

0
2

Proposed
KM−CKSCKS−KM
KM
CKS
4

6
8
number of domains: D

10

Figure 4: Adjusted Rand index with different numbers of
domains D using the Synth5 data set.
However, the proposed method consistently achieved thehighest performance for each number of domains.
The computational time for inference of the proposedmodel with the Synth5 data set was 18 seconds. The demanding part of the inference is E-step. The computational
complexity of each E-step is O(N JK 3 ), where N is the total number of objects, J is the number of clusters, and K is
the latent dimensionality.

6 Conclusion
We proposed a generative model approach for unsupervised
cluster matching, which we call the many-to-many matchinglatent variable model. In experiments, we confirmed that the
proposed model can perform much better than object matching, clustering and their combinations. Advantages of the
proposed model over the existing methods are that it canfind many-to-many matching, and can handle multiple domains with different numbers of objects. Because the proposed model is a probabilistic generative model, we can extend it in a probabilistically principled manner.
Although our results have been encouraging as a firststep towards unsupervised object clustering, we must extend our approach in a number of directions. First, we would
like to extend the proposed model to other types of data.
The proposed model assumes real values with Gaussian
noise for features. However, the features can be discretevalues for bag-of-features represented images and annotations for example. We can handle discrete values in the proposed framework by incorporating topic models (Blei, Ng,
and Jordan 2003). The proposed model assumes the linearity of features with respect to their latent vectors. We
can relax this assumption by using nonlinear matrix factorization techniques (Lawrence and Urtasun 2009). Second,
we would like to evaluate the proposed model in a semiunsupervised setting (Kimura et al. 2010), where a small
number of object correspondences over different domains
are available. The information can assist the matching by incorporating a condition stating that the cluster assignmentsof the corresponding objects become the same. Finally, we
would like to use the proposed method for real applications,which include image annotation (Socher and Fei-Fei 2010),
cross domain recommendation (Li, Yang, and Xue 2009),
multi-lingual corpus analysis (Boyd-Graber and Blei 2009;
Iwata, Mochihashi, and Sawada 2010), machine translation(Haghighi et al. 2008), and bioinformatics (Wang and Mahadevan 2008).

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