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Nowadays there is an important need by journalists and media monitoring companies to cluster news in large amounts of web articles, in order to ensure fast access to their topics or events of interest. Our aim in this work is to identify groups of news articles that share a common topic or event, without a priori knowledge of the number of clusters. The estimation of the correct number of topics is a challenging issue, due to the existence of “noise”, i.e. news articles which are irrelevant to all other topics. In this context, we introduce a novel density-based news clustering framework, in which the assignment of news articles to topics is done by the well-established Latent Dirichlet Allocation, but the estimation of the number of clusters is performed by our novel method, called “DBSCAN-Martingale”, which allows for extracting noise from the dataset and progressively extracts clusters from an OPTICS reachability plot. We evaluate our framework and the DBSCAN-Martingale on the 20newsgroups-mini dataset and on 220 web news articles, which are references to specific Wikipedia pages. Among twenty methods for news clustering, without knowing the number of clusters k, the framework of DBSCAN-Martingale provides the correct number of clusters and the highest Normalized Mutual Information.
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A hybrid framework for news clustering based
on the DBSCAN-Martingale and LDA
Ilias Gialampoukidis, Stefanos Vrochidis, and Ioannis Kompatsiaris
Information Technologies Institute, CERTH, Thessaloniki, Greece
Abstract. Nowadays there is an important need by journalists and me-
dia monitoring companies to cluster news in large amounts of web arti-
cles, in order to ensure fast access to their topics or events of interest. Our
aim in this work is to identify groups of news articles that share a com-
mon topic or event, without a priori knowledge of the number of clusters.
The estimation of the correct number of topics is a challenging issue, due
to the existence of “noise”, i.e. news articles which are irrelevant to all
other topics. In this context, we introduce a novel density-based news
clustering framework, in which the assignment of news articles to top-
ics is done by the well-established Latent Dirichlet Allocation, but the
estimation of the number of clusters is performed by our novel method,
called “DBSCAN-Martingale”, which allows for extracting noise from
the dataset and progressively extracts clusters from an OPTICS reacha-
bility plot. We evaluate our framework and the DBSCAN-Martingale on
the 20newsgroups-mini dataset and on 220 web news articles, which are
references to specific Wikipedia pages. Among twenty methods for news
clustering, without knowing the number of clusters k, the framework of
DBSCAN-Martingale provides the correct number of clusters and the
highest Normalized Mutual Information.
Keywords: Clustering News Articles, Latent Dirichlet Allocation, DBSCAN-
Martingale
1 Introduction
Clustering news articles is a very important problem for journalists and me-
dia monitoring companies, because of their need to quickly detect interesting
articles. This problem becomes also very challenging and complex, given the rel-
atively large amount of news articles produced on a daily basis. The challenges
of the aforementioned problem are summarized into two main directions: (a)
discover the correct number of news clusters and (b) group the most similar
news articles into news clusters. We face these challenges under the following
assumptions. Firstly, we take into account that real data is highly noisy and
the number of clusters is not known. Secondly, we assume that there is a lower
bound for the minimum number of documents per cluster. Thirdly, we consider
the names/labels of the clusters unknown.
This is a draft version of the paper. The final version is available at:
http://link.springer.com/chapter/10.1007/978-3-319-41920-6_13
P. Perner (Ed): MLDM 2016, Machine Learning and Data Mining in Pattern Recognition, LNAI
pp. 170–184, 2016. DOI: 10.1007/978-3-319-41920-6_13
Towards addressing this problem, we introduce a novel hybrid clustering
framework for news clustering, which combines automatic estimation of the num-
ber of clusters and assignment of news articles into topics of interest. The estima-
tion of the number of clusters is done by our novel “DBSCAN-Martingale”, which
can deal with the aforementioned assumptions. The main idea is to progres-
sively extract all clusters (extracted by a density-based algorithm) by applying
Doob’s martingale and then apply a well-established method for the assignment
of news articles to topics, such as Latent Dirichlet Allocation (LDA). The pro-
posed hybrid framework does not consider known the number of news clusters,
but requires only the more intuitive parameter minP ts, as a lower bound for
the number of documents per topic. Each realization of the DBSCAN-Martingale
provides the number of detected topics and, due to randomness, this number is
a random variable. As the final number of detected topics, we use the major-
ity vote over 10 realizations of the DBSCAN-Martingale. Our contribution is
summarized as follows:
We present our novel DBSCAN-Martingale process, which progressively es-
timates the number of clusters in a dataset.
We introduce a novel hybrid news clustering framework, which combines our
DBSCAN-Martingale with Latent Dirichlet Allocation.
In the following, we present, in Section 2, existing approaches for news clus-
tering and density-based clustering. In Section 3, we propose a new hybrid frame-
work for news clustering, where the number of news clusters is estimated by our
“DBSCAN-Martingale”, which is presented in Section 4. Finally, in Section 5,
we test both our novel method for estimating the number of clusters and our
news clustering framework in four datasets of various sizes.
2 Related Work
News clustering is tackled as a text clustering problem [1], which usually involves
feature selection [25], spectral clustering [21] and k-means oriented [1] techniques,
assuming mainly that the number of news clusters is known. We consider the
more general and realistic case, where the number of clusters is unknown and it
is possible to have news articles which do not belong to any of the clusters.
Latent Dirichlet Allocation (LDA) [4] is a popular model for topic model-
ing, given the number of topics k. LDA has been generalized to nonparametric
Bayesian approaches, such as the hierarchical Dirichlet process [29] and DP-
means [20], which predict the number of topics k. The extraction of the correct
number of topics is equivalent to the estimation of the correct number of clus-
ters in a dataset. The majority vote among 30 clustering indices has recently
been proposed in [7] as an indicator for the number of clusters in a dataset. In
contrast, we propose an alternative majority vote among 10 realizations of the
“DBSCAN-Martingale”, which is a modification of the DBSCAN algorithm [12]
and has three main advantages and characteristics: (a) they discover clusters
with not-necessarily regular shapes, (b) they do not require the number of clus-
ters and (c) they extract noise. The parameters of DBSCAN are the density level
and a lower bound for the minimum number of points per cluster: minP ts.
Other approaches for clustering that could be applied to news clustering,
without knowing the number of clusters, are based on density based cluster-
ing algorithms. The graph-analogue of DBSCAN has been presented in [5] and
dynamically adjusting the density level , the nested hierarchical sequence of
clusterings results to the HDBSCAN algorithm [5]. OPTICS [2] allows for de-
termining the number of clusters in a dataset by counting the “dents” of the
OPTICS reachability plot. F-OPTICS [28] has reduced the computational cost
of the OPTICS algorithm using a probabilistic approach of the reachability dis-
tance, without significant accuracy reduction. The OPTICS-ξalgorithm [2] re-
quires an extra parameter ξ, which has to be manually set in order to find “dents”
in the OPTICS reachability plot. The automatic extraction of clusters from the
OPTICS reachability plot, as an extension of the OPTICS-ξalgorithm, has been
presented in [27] and has been outperformed by HDBSCAN-EOM [5] in several
datasets. We will examine whether some of these density based algorithms per-
form well on the news clustering problem and we shall compare them with our
DBSCAN-Martingale, which is a modification of DBSCAN, where the density
level is a random variable and the clusters are progressively extracted.
3 The DBSCAN-Martingale framework for news
clustering
We propose a novel framework for news clustering, where the number of clusters
kis estimated using the DBSCAN-Martingale and documents are assigned to k
topics using Latent Dirichlet Allocation (LDA).
Feature
Extraction
DBSCAN-
Martingale
LDA(k)
Text
documents
k topics
Bi-grams
Uni-grams
topic 1
topic 2
topic k
Fig. 1: Our hybrid framework for news clustering, using the DBSCAN-Martingale
and Latent Dirichlet Allocation.
We combine DBSCAN and LDA because LDA performs well on text clus-
tering but requires the number of clusters. On the other hand, density-based
algorithms do not require the number of clusters, but their performance in text
clustering is limited, when compared to LDA.
LDA [4] is a probabilistic topic model, which assumes a Bag-of-Words repre-
sentation of the collection of documents. Each topic is a distribution over terms in
a fixed vocabulary, which assigns probabilities to words. Moreover, LDA assumes
that documents exhibit multiple topics and assigns a probability distribution on
the set of documents. Finally, LDA assumes that the order of words does not
matter and, therefore, is not applicable to word n-grams for n2.
We refer to word n-grams as “uni-grams” for n= 1 and as “bi-grams” for
n= 2. The DBSCAN-Martingale performs well on the bi-grams, following the
concept of “phrase extraction” [1]. We restrict our study on textual features
(n-grams) in the present work and spatiotemporal features are not used.
In Figure 1 the estimation of the number of clusters is done by DBSCAN-
Martingale and LDA follows for the assignment of text documents to clusters.
4 DBSCAN-Martingale
In this Section, we show the construction of the DBSCAN-Martingale. In Section
4.1 we provide the necessary background in density-based clustering and the
notation which we adopt. In Section 4.2, we progressively estimate the number
of clusters in a dataset by defining a stochastic process, which is then shown
(Section 4.3) to be a Martingale process.
4.1 Notation and Preliminaries on DBSCAN
Given a dataset of n-points, density-based clustering algorithms provide as out-
put the clustering vector C. Assuming there are kclusters in the dataset, some
of the points are assigned to a cluster and some points do not belong to any of
the kclusters. When a point j= 1,2, . . . , n is assigned to one of the kclusters,
the j-th element of the clustering vector C, denoted by C[j] takes the value
of the cluster ID from the set {1,2, . . . , k}. Otherwise, the j-th point does not
belong to any cluster, it is marked as “noise” and the corresponding value in the
clustering vector becomes zero, i.e. C[j] = 0. Therefore, the clustering vector C
is a n-dimensional vector with values in {0,1,2, . . . , k}.
The algorithm DBSCAN [12] is a density-based algorithm, which provides
one clustering vector, given two parameters, the density level and the parameter
minP ts. We denote the clustering vector provided by the algorithm DBSCAN
by CDBSC AN(,minP ts)or simply CDBSC AN()because the parameter minP ts is
considered as a pre-defined fixed parameter. For low values of ,CDBSCAN ()is
a vector of zeros (all points are marked as noise). On the other hand, for high
values of ,CDBSC AN()is a column vector of ones. Apparently, if a clustering
vector has only zeros and ones, only one cluster has been detected and the
partitioning is trivial.
−0.5 0.0 0.5 1.0 1.5 2.0 2.5
0.0 0.5 1.0 1.5 2.0 2.5
(a) Data
0 100 200 300 400 500
0.1 0.2 0.3 0.4 0.5 0.6
OPTICS order
Reachability distance
c1
c2
c3c4
c5
(b) OPTICS clusters
Fig. 2: OPTICS reachability plot and randomly generated density levels
Clusters detected by DBSCAN strongly depend on the density level . An
indicative example is shown in Figure 2(a), where the 5 clusters do not have
the same density, and it is evident that there is no single value of that can
output all the clusters. In Figure 2(b), we illustrate the corresponding OPTICS
reachability plot with 5 randomly selected density levels (horizontal dashed lines)
and none of them is able to extract all clusters C1, C2, . . . , C5into one clustering
vector C.
In order to deal with this problem we introduce (in Sections 4.2 and 4.3) an
extension of DBSCAN based on Doob’s Martingale, which allows for introducing
a random variable and involves the construction of a Martingale process, which
progressively approaches the clustering vector which contains all clusters so as
to determine the number of clusters.
4.2 Estimation of the number of clusters with the
DBSCAN-Martingale
We introduce a probabilistic method to estimate the number of clusters, by
constructing a martingale stochastic process [11], which is able to progressively
extract all clusters for all density levels. The martingale construction is, in gen-
eral, based on Doob’s martingale [11], in which we progressively gain knowledge
about the result of a random variable. In the present work, the random variable
that needs to be known is the vector of cluster IDs, which is a combination of T
clustering vectors CDBSC AN(t), t = 1,2, . . . , T .
First, we generate a sample of size Twith random numbers t, t = 1,2, . . . , T
uniformly in [0, max], where max is an upper bound for the density levels. The
sample of t, t = 1,2, . . . , T is sorted in increasing order and the values of t
can be demonstrated on an OPTICS reachability plot, as shown in Figure 2
(T= 5). For each density level twe find the corresponding clustering vectors
CDBS CAN (t)for all stages t= 1,2, . . . , T .
In the beginning of the algorithm, there are no clusters detected. In the first
stage (t= 1), all clusters detected by CDBSC AN(1)are kept, corresponding to
the lowest density level 1. In the second stage (t= 2), some of the detected
clusters by CDBSC AN(2)are new and some of them have also been detected at
previous stage (t= 1). In order to keep only the newly detected clusters of the
second stage (t= 2), we keep only groups of numbers of the same cluster ID
with size greater than minP ts.
 
  
 
 
Update the labels of the clusters
Update the vector     
 
Update the number of clusters:
   
  
  by definition
New cluster detected at
Fig. 3: One realization of the DBSCAN-Martingale with T= 2 iterations. The
points with cluster label 2in C(1) are re-discovered as a cluster by CDBSC AN(2)
but the update rule keeps only the newly detected cluster.
Formally, we define the sequence of vectors C(t), t = 1,2, . . . , T , where C(1) =
CDBS CAN (1)and:
C(t)[j] := 0 if point jbelongs to a previously extracted cluster
CDBS CAN (t)[j] otherwise
(1)
Since the stochastic process C(t), t = 1,2, . . . , T is a martingale, as shown in
Section 4.3, and CDBS CAN (t)is the output of DBSCAN for the density level t,
the proposed method is called “DBSCAN-Martingale”.
Finally, we relabel the cluster IDs. Assuming that rclusters have been de-
tected for the first time at stage t, we update the cluster labels of C(t)starting
from 1 + maxjC(t1)[j] to r+ maxjC(t1) [j]. Note that the maximum value of
a clustering vector coincides with the number of clusters.
The sum of all vectors C(t)up to stage Tis the final clustering vector of our
algorithm:
C=C(1) +C(2) +· · · C(T)(2)
The estimated number of clusters ˆ
kis the maximum value of the final clus-
tering vector C:
ˆ
k= max
jC[j] (3)
In Figure 3, we adopt the notation XTfor the transpose of the matrix or
vector X, in order to demonstrate the estimation of the number of clusters after
two iterations of the DBSCAN-Martingale.
The process we have formulated, namely the DBSCAN-Martingale, is repre-
sented as pseudo code in Algorithm 1. Algorithm 1 extracts clusters sequentially,
combines them into one single clustering vector and outputs the most updated
estimation of the number of clusters ˆ
k.
Algorithm 1: DBSCAN-Martingale(minP ts)return ˆ
k
1: Generate a random sample of Tvalues in [0, max]
2: Sort the generated sample t, t = 1,2,...,T
3: for t= 1 to T
4: find CDBS CAN(t)
5: compute C(t)as in Eq. (1)
6: update the cluster IDs
7: update the vector Cas in Eq. (2)
8: update ˆ
k= maxjC[j]
9: end for
10: return ˆ
k
The DBSCAN-Martingale requires Titerations of the DBSCAN algorithm,
which runs in O(nlog n) if a tree-based spatial index can be used and in O(n2)
without tree-based spatial indexing [2]. Therefore, the DBSCAN-Martingale runs
in O(T n log n) for tree-based indexed datasets and in O(T n2) without tree-based
indexing. Our code is written in R1, using the dbscan2package, which runs
DBSCAN in O(nlog n) with kd-tree data structures for fast nearest neighbor
search.
The DBSCAN-Martingale (one execution of Algorithm 1) is illustrated, for
example, on the OPTICS reachability plot of Figure 2 (b) where, for the ran-
dom sample of density levels t, t = 1,2,...,5 (horizontal dashed lines), we
sequentially extract all clusters. In the first density level 1= 0.12, DBSCAN-
Martingale extracts the clusters C1, C3and C4, but in the density level 2= 0.21
1https://www.r-project.org/
2https://cran.r-project.org/web/packages/dbscan/index.html
3 4 5 6
clusters
probability
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
(a) number of clusters
10 20 30 40 50 60 70 80
4 5 6 7 8 9
minPts
number of clusters
(b) robustness to the param-
eter minP ts
t
clusters extracted
12345678910
345
(c) Convergence to k(one
realization)
Fig. 4: The number of clusters as generated by DBSCAN-Martingale (minP ts =
50) after 100 realizations
Algorithm 2: MajorityVote(realizations, minP ts)return ˆ
k
1: clusters =,k= 0
2: for r= 1 to realizations
3: k=DBSCAN-Martingale(minP ts)
4: clusters = AppendTo(clusters, k)
5: end for
6: ˆ
k= mode(clusters)
7: return ˆ
k
no new clusters are extracted. In the third density level, 3= 0.39, the clusters
C2and C5are added to the final clustering vector and in the other density
levels, 4and 5there are no new clusters to extract. The number of clusters
extracted up to stage tis shown in Figure 4(c). Observe that at t= 3 iterations,
DBSCAN-Martingale has output k= 5 and for all iterations t > 3 there are no
more clusters to extract. Increasing the total number of iterations Twill need-
lessly introduce additional computational cost in the estimation of the number
clusters ˆ
k.
The estimation of number of clusters ˆ
kis a random variable, because it in-
herits the randomness of the density levels t, t = 1,2, . . . , T . For each execution
of Algorithm 1, one realization of the DBSCAN-Martingale generates ˆ
k, so we
propose as the final estimation of the number of clusters the majority vote over
10 realizations of the DBSCAN-Martingale.
Algorithm 2 outputs the majority vote over a fixed number of realizations of
the DBSCAN-Martingale. For each realization, the estimated number of clusters
kis added to the list clusters and the majority vote is obtained from the mode
of clusters, since the mode is defined as the most frequent value in a list. The
percentage of realizations where the DBSCAN-Martingale outputs exactly ˆ
k
clusters is a probability distribution, such as the one shown in Figure 4(a), which
corresponds to the illustrative dataset of Figure 2(a). Finally, we note that the
same result (ˆ
k= 5) appears for a wide range of the parameter minP ts (Figure
4(b)), a fact that demonstrates the robustness of our approach.
4.3 The sequence of vectors C(t)is a martingale process
Martingale is a random process X1, X2, . . . for which the expected future value
of Xt+1, given all prior values X1, X2, . . . , Xt, is equal to the present observed
value Xt. Doob’s martingale is a generic martingale construction, in which our
knowledge about a random variable is progressively obtained:
Definition 1. (Doob’s Martingale) [11]. Let X, Y1, Y2, . . . be any random vari-
ables with E[|X|]<. Then if Xtis defined by Xt=E[X|Y1, Y2, . . . , Yt], the
sequence of Xt, t = 1,2, . . . is a martingale.
In this context, we will show that the sequence of clustering vectors Xt=
C(1) +C(2) +· · · +C(t), t = 1,2, . . . , T is Doob’s martingale for the sequence of
random variables Yt=CDBSC AN(t), t = 1,2, . . . , T .
We denote by < Zi, Zl>=PjZi[j]·Zl[j] the inner product of any two
vectors Ziand Zland we prove the following Lemma:
Lemma 1. If two clustering vectors Zi, Zlare mutually orthogonal, they contain
different clusters.
Proof. The values of the clustering vectors are cluster IDs so they are non-
negative integers. Points which do not belong to any of the clusters (noise) are
assigned zeros. Since < Zi, Zl>=PjZi[j]·Zl[j] = 0 and based on the fact that
when a sum of non-negative integers is zero, then all integers are zero, we obtain
Zi[j] = 0 or Zl[j] = 0 for all j= 1,2, . . . , n.
For example, the clustering vectors
Zi=[00000000000000010001110101100000022222]T
Zl=[11011110101000000000000000000000000000]T
are mutually orthogonal and contain different clusters.
Martingale construction. Each density level t, t = 1,2, . . . , T provides one
clustering vector CDBSC AN(t)for all t= 1,2, . . . , T . As tincreases, more clus-
tering vectors are computed and we gain knowledge about the vector C.
In Eq. (1), we constructed a sequence of vectors C(t), t = 1,2, . . . , T , where
each C(t)is orthogonal to all C(1), C (2), . . . , C(t1) , from Lemma 1. The sum of
all clustering vectors C(1) +C(2) +. . . +C(t1) has zeros as cluster IDs in the
points which belong to the clusters of C(t). Therefore, C(t)is also orthogonal
to C(1) +C(2) +. . . +C(t1) . We use the orthogonality to show that the vector
C(1) +C(2) +. . . +C(t)is our “best prediction” for the final clustering vector C
at stage t. The expected final clustering vector at stage tis:
E[C|CDBS CAN (1), CDBS CAN (2), . . . , CDBSC AN(t)] = C(1) +C(2) +. . . +C(t).
Initially, the final clustering Cvector is the zero vector O. Our knowledge
about the final clustering vector up to stage tis restricted to C(1)+C(2) +. . .+C(t)
and finally, at stage t=T, we have gained all available knowledge about the final
clustering vector C, i.e. C=E[C|CDBSC AN(1), CDBSC AN(2), . . . , CD BSC AN(T)].
5 Experiments
5.1 Dataset description
The proposed methodology is evaluated on the 20newsgroups-mini dataset with
2000 articles, which is available on the UCI repository3and on 220 news arti-
cles, which are references to specific Wikipedia pages so as to ensure reliable
ground-truth: the WikiRef220. We also use two subsets of WikiRef220, namely
the WikiRef186 and the WikiRef150, in order to test DBSCAN-Martingale in
four datasets of sizes 2000, 220, 150 and 115 documents respectively.
We selected these datasets because we focus on datasets with news clusters
which are event-oriented, like “Paris Attacks November 2016” or they discuss
about specific topics like “Barack Obama” (rather than “Politics” in general).
We would tackle the news categorization problem as a supervised classification
problem, because training sets are available, contrary to the news clustering
problem where, for example, the topic “Malaysia Airlines Flight 370” had no
training set before the 8th of March 2014.
We assume that 2000 news articles is a reasonable upper bound for the num-
ber of recent news articles that can be considered for news clustering, in line with
other datasets that were used to evaluate similar methods [25, 5]. In all datasets
(Table 2) we extract uni-grams and bi-grams, assuming a Bag-of-Words repre-
sentation of text. Before the extraction of uni-grams and bi-grams, we remove
the SMART4stopwords list and we then stem the words using Porter’s algo-
rithm [24]. The uni-grams are filtered out if they occur less than 6 times and
the bi-grams if they occur less than 20 times. The final bi-grams are normalized
using tf-idf weighting and, in all datasets, the upper bound for the density level
is taken max = 3. We generate a sample of T= 5 uniformly distributed numbers
using R, for the initialization of Algorithm 1.
Table 1: DBSCAN results without LDA, for the 5 best values of and minP ts.
The DBSCAN-Martingale requires no tuning for determining and is able to
extract all clusters for datasets (eg. WikiRef220) in which there is no unique
density level to extract all clusters.
WikiRef150 WikiRef186 WikiRef220 20news
clusters NMI clusters NMI clusters NMI clusters NMI
0.8 3 0.3850 0.8 3 0.3662 0.8 3 0.3733 1.6 20 0.0818
0.9 4 0.4750 0.9 3 0.4636 0.9 3 0.4254 1.7 20 0.0818
1.0 3 0.4146 1.0 4 0.4904 1.0 4 0.5140 1.8 20 0.0818
1.1 3 0.4234 1.1 3 0.3959 1.1 3 0.4060 1.9 20 0.0818
1.2 1 0.1706 1.2 2 0.1976 1.2 3 0.4124 2.0 20 0.0818
3http://archive.ics.uci.edu/ml/datasets.html
4http://jmlr.csail.mit.edu/papers/volume5/lewis04a/a11-smart-stop-
list/english.stop
Table 2: Estimated number of topics. The best values are marked in bold. The
majority rule for 10 realizations of the DBSCAN-Martingale coincides with the
ground truth number of topics.
Index Ref WikiRef150 WikiRef186 WikiRef220 20news
CH [6] 30 29 30 30
Duda [9] 2 2 2 2
Pseudo t2[9] 2 2 2 2
C-index [17] 27 2 2 2
Ptbiserial [8] 11 7 6 30
DB [8] 2 46 2
Frey [13] 2 2 2 5
Hartingan [15] 18 20 16 24
Ratkowsky [26] 20 24 29 30
Ball [3] 33 3 3
McClain [22] 2 2 2 2
KL [19] 14 15 17 15
Silhouette [18] 30 44 2
Dunn [10] 2 4 5 3
SDindex [14] 4 46 3
SDbw [14] 30 7 6 3
NbClust [7] 2 2 6 2
DP-means [20] 4 47 15
HDBSCAN-EOM [5] 5 5 536
DBSCAN-Martingale 3 4 5 20
5.2 Evaluation
The evaluation of our method is done in two levels. Firstly, we test whether
the output of the majority vote over 10 realizations of the DBSCAN-Martingale
matches the ground-truth number of clusters. Secondly, we evaluate the over-
all hybrid news clustering framework, using the number of clusters from Table
2. The index “NbClust”, which is computed using the NbClust5package, is the
majority vote among the 24 indices: CH, Duda, Pseudo t2, C-index, Beale, CCC,
Ptbiserial, DB, Frey, Hartigan, Ratkowsky, Scott, Marriot, Ball, Trcovw, Tracew,
Friedman, McClain, Rubin, KL, Silhouette, Dunn, SDindex, SDbw [7]. The Din-
dex and Hubert’s Γare graphical methods and they are not involved in the
majority vote. The indices GAP, Gamma, Gplus and Tau are also not included
in the majority vote, due to the high computational cost. The NbClust package
requires as a parameter the maximum number of clusters to look for, which is
set max.nc = 30. For the extraction of clusters from the HDBSCAN hierarchy,
we adopt the EOM-optimization [5] and for the nonparametric Bayesian method
DP-means, we extended the R-script which is available on GitHub6.
5https://cran.r-project.org/web/packages/NbClust/index.html
6https://github.com/johnmyleswhite/bayesian nonparametrics/tree/master/code/dp-
means
1 2 3 4
clusters
probability
0.0 0.1 0.2 0.3 0.4 0.5
(a) WikiRef150
10 15 20 25 30
1 2 3 4 5
minPts
number of clusters
(b) minP ts
12345
clusters
probability
0.0 0.1 0.2 0.3 0.4
(c) WikiRef186
10 15 20 25 30
1 2 3 4 5
minPts
number of clusters
(d) minP ts
123456
clusters
probability
0.00 0.05 0.10 0.15 0.20 0.25
(e) WikiRef220
10 15 20 25
2.0 2.5 3.0 3.5 4.0 4.5 5.0
minPts
number of clusters
(f) minP ts
18 19 20 21 22 23
clusters
probability
0.00 0.10 0.20 0.30
(g) 20news
5 10 15 20
10 20 30 40
minPts
number of clusters
(h) minP ts
Fig. 5: The number of clusters as generated by DBSCAN-Martingale
Evaluation of the number of clusters: We compare our DBSCAN-Martingale
with baseline methods, listed in Table 2, which either estimate the number of
clusters directly, or provide a clustering vector without any knowledge of the
number of clusters. The Ball index is correct in the WikiRef150 dataset, HDB-
SCAN and Dunn is correct in the WikiRef220 dataset and the indices DB, Sil-
houette, Dunn, SDindex and DP-means are correct in the WikiRef186 datasets.
However, in all datasets, the estimation given by the majority vote over 10 re-
alizations of the DBSCAN-Martingale coincides with the ground truth number
of clusters. In Figure 5, we present the estimation of the number of clusters for
100 realizations of the DBSCAN-Martingale, in order to show that after 10 runs
of 10 realizations the output of Algorithm 1 remains the same. The parameter
minP ts is taken equal to 10 for the 20news dataset and 20 for all other cases.
In all datasets examined, we observe that there are some samples of density
levels t, t = 1,2, . . . , T which do not provide the correct number of clusters (Fig-
ure 5). The “mis-clustered” samples are due to the randomness of the density
levels t, which are sampled from the uniform distribution. We expect that sam-
pling from another distribution would result to less mis-clustered samples, but
searching for the statistical distribution of tis beyond the scope of this paper.
We also compared the DBSCAN-Martingale with several methods of Table
2, with respect to the mean processing time. All experiments were performed
on an Intel Core i7-4790K CPU at 4.00GHz with 16GB RAM memory, using a
single thread and the R statistical software. Given a corpus of 500 news arti-
cles, DBSCAN-Martingale run in 0.39 seconds, while the Duda, Pseudo t2and
Table 3: Normalized Mutual Information after LDA by kclusters, where kis
estimated in Table 2. The standard deviation is provided for 10 runs and the
highest values are marked in bold.
Index + LDA WikiRef150 WikiRef186 WikiRef220 20news
CH 0.5537 (0.0111) 0.6080 (0.0169) 0.6513 (0.0126) 0.3073 (0.0113)
Duda 0.6842 (0.0400) 0.6469 (0.0271) 0.6381 (0.0429) 0.1554 (0.0067)
Pseudo t20.6842 (0.0400) 0.6469 (0.0271) 0.6381 (0.0429) 0.1554 (0.0067)
C-index 0.5614 (0.0144) 0.6469 (0.0271) 0.6381 (0.0429) 0.1554 (0.0067)
Ptbiserial 0.6469 (0.0283) 0.6469 (0.0271) 0.8262 (0.0324) 0.3073 (0.0113)
DB 0.6842 (0.0400) 0.7892 (0.0553) 0.8262 (0.0324) 0.1554 (0.0067)
Frey 0.6842 (0.0400) 0.6469 (0.0271) 0.6381 (0.0429) 0.2460 (0.0198)
Hartingan 0.5887 (0.0157) 0.6513 (0.0184) 0.7156 (0.0237) 0.3126 (0.0098)
Ratkowsky 0.5866 (0.0123) 0.6201 (0.0188) 0.6570 (0.0107) 0.3073 (0.0113)
Ball 0.7687 (0.0231) 0.7655 (0.0227) 0.7601 (0.0282) 0.2101 (0.0192)
McClain 0.6842 (0.0400) 0.6469 (0.0271) 0.6381 (0.0429) 0.1554 (0.0067)
KL 0.6097 (0.0232) 0.6670 (0.0156) 0.7091 (0.0257) 0.3077 (0.0094)
Silhouette 0.5537 (0.0111) 0.7892 (0.0553) 0.8032 (0.0535) 0.1554 (0.0067)
Dunn 0.5805 (0.0240) 0.7892 (0.0553) 0.8560 (0.0397) 0.2101 (0.0192)
SDindex 0.7007 (0.0231) 0.7892 (0.0553) 0.8262 (0.0324) 0.2101 (0.0192)
SDbw 0.5537 (0.0111) 0.7668 (0.0351) 0.8262 (0.0324) 0.2101 (0.0192)
NbClust 0.6842 (0.0400) 0.6469 (0.0271) 0.8262 (0.0324) 0.1554 (0.0067)
DP-means 0.7007 (0.0231) 0.7892 (0.0553) 0.8278 (0.0341) 0.3077 (0.0094)
HDBSCAN-EOM 0.7145 (0.0290) 0.7630 (0.0530) 0.8560 (0.0397) 0.3106 (0.0134)
DBSCAN-Martingale 0.7687 (0.0231) 0.7892 (0.0553) 0.8560 (0.0397) 0.3137 (0.0130)
Dunn in 0.44 seconds, SDindex in 1.06 seconds, HDBSCAN in 1.23 seconds and
Silhouette in 1.37 seconds.
Evaluation of news clustering: The evaluation measure is the popular
Normalized Mutual Information (NMI), mainly used for the evaluation of clus-
tering techniques, which allows us to compare results when the number of out-
putted clusters does not match the number of clusters in the ground truth [20].
For the output kof each method of Table 2, we show the average of 10 runs of
LDA (and the corresponding standard deviation) in Table 3. For the WikiRef150
dataset, the combination of Ball index with LDA provides the highest NMI. For
the WikiRef220 dataset, the combinations of HDBSCAN with LDA and Dunn
index with LDA also provide the highest NMI. For the WikiRef186 dataset, the
combinations of LDA with the indices DB, Silhouette, Dunn, SDindex and DP-
means perform well. However, in all 4 datasets, our news clustering framework
provides the highest NMI score and in the case of 20news dataset, the combina-
tion of DBSCAN-Martingale with LDA is the only method which provides the
highest NMI score. Without using LDA, the best partition provided by DBSCAN
has NMI less than 51.4 % in all WikiRef150, WikiRef186 and WikiRef220, as
shown in Table 1. In contrast, we adopt the LDA method which achieves NMI
scores up to 85.6 %. Density-based algorithms such as DBSCAN, HDBSCAN
and DBSCAN-Martingale assigned too much noise in our datasets, a fact that
affected the clustering performance, especially when compared to LDA in news
clustering, thus we kept only the estimation ˆ
k.
6 Conclusion
We have presented a hybrid framework for news clustering, based on the DBSCAN-
Martingale for the estimation of the number of news clusters, followed by the
assignment of the news articles to topics using Latent Dirichlet Allocation. We
extracted the word n-grams of a news articles collection and we estimated the
number of clusters, using the DBSCAN-Martingale which is robust to noise. The
extension of the DBSCAN algorithm, based on the martingale theory, allows for
introducing a variable density level in the clustering algorithm. Our method out-
performs several state-of-the-art methods on 4 corpora, in terms of the number
of detected clusters, and the overall news clustering framework shows a good
behavior of the proposed martingale approach, as evaluated by the Normalized
Mutual Information. In the future, we plan to evaluate our framework using
alternatice to LDA text clustering approaches, additional features and content,
in order to present the multimodal and multilingual version of our framework.
Acknowledgements
This work was supported by the projects MULTISENSOR (FP7-610411) and
KRISTINA (H2020-645012), funded by the European Commission.
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