PACLIC 24 Proceedings

Multi-aspects Review Summarization Based on Identification ofImportant Opinions and their Similarity ?
Ryosuke Tadano, Kazutaka Shimada, and Tsutomu Endo
Department of Artificial Intelligence, Kyushu Institute of Technology,
680-4 Kawazu, Iizuka, Fukuoka 820-8502, Japan{r tadano, shimada, endo}@pluto.ai.kyutech.ac.jp

Abstract. The development of the Web services lets many users easily provide their opinions
recently. Automatic summarization of enormous sentiments has been expected. Intuitively,
we can summarize a review with traditional document summarization methods. However,such methods have not well-discussed “aspects”. Basically, a review consists of sentimentswith various aspects. We summarize reviews for each aspect so that the summary presents
information without biasing to a specific topic. In this paper, we propose a method for multiaspects review summarization based on evaluative sentence extraction. We handle three features; ratings of aspects, the tf -idf value, and the number of mentions with a similar topic.
For estimating the number of mentions, we apply a clustering algorithm. By integratingthese features, we generate a more appropriate summary. The experiment results show theeffectiveness of our method.
Keywords: sentiment analysis, multi-aspects review summarization, ratings of aspects, important sentence extraction, opinion integration.

1 Introduction
As Web services like CGMs have become widely used, people can easily post reviews for products
or services. Although handling these information (evaluative information) has become necessary,
there exists too much information on the Web. Therefore, extracting information that users wantand summarizing them have been expected recently. Intuitively, we can summarize a review withtraditional document summarization methods. For instance, Brandow et al. (1995) have summarized a document by extracting sentences with some features such as the presence of signaturewords and the location in the document. For sentiment summarization, Pang and Lee (2004) haveextracted all subjective sentences. They suggested that these extracted sentences could be used assummaries. However, a review basically consists of sentiments with various aspects (i.e., “image
quality” and “usability” of a camera). Therefore, we need to extract information for each aspect in
the case of review summarization. Aspect summarization can present information without biasing
to a specific topic. We focus on multi-aspects review summarization in this research.
We generate a summary by extracting important sentences and arranging them. Since we extract summary sentences, we need to discuss which sentences are important and how to extract
important sentences. In the case of treating multi-reviews, we need to handle the redundant information. Pang and Lee (2008) have reported that while in traditional summarization redundant information is discarded, in sentiment summarization redundancy indicates the importanceof opinions. Therefore, we treat redundancy as a feature for decision of important sentences. We
assume that reviews we treat in this research have multiple aspects and a reviewer gives ratings ofthe aspects (i.e., 0 to 5 stars). Reviewers also write free comments about the target. We leverage?

This work was supported by Kayamori Foundation of Informational Science Advancement
Copyright 2010 by Ryosuke Tadano, Kazutaka Shimada, and Tsutomu Endo

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Figure 1: The outline of our method.

three features: ratings of aspects of reviews, the tf -idf value, and the number of mentions witha similar topic. We apply a clustering algorithm to sentences to measure the number of mentions
with a similar topic. Then, we aim to integrate those features and generate a more appropriate
summary. Figure 1 shows the outline of our method.

2 Features for sentence extraction
For generating a summary, we define the following three features: (1) ratings of aspects, (2) the
tf -idf value, and (3) the number of mentions with a similar topic. The following sections describe
how we treat these features.

2.1 Ratings of aspects
If we generate a summary, the summary needs to contain the proper balance of whole opinions
in the reviews. For instance, if we summarize only positive opinions, the summary cannot tellreaders negative opinions of a target.
We focus on ratings of aspects given to reviews as a feature to deal with this problem. We
assume that a reviewer writes a comment corresponding to the rating. For instance, if a reviewer
gives 1 star for one aspect, the reviewer writes negative comments for the aspect. We assign ratingsof aspects of a review to evaluative sentences included in the review. Each evaluative sentence has
the rating corresponding to the aspect of the sentence. We use the pair of the evaluative sentence
and the rating to consider the distribution of the ratings for summarization.

2.2 The tf -idf value
The tf -idf algorithm is used as a major algorithm for many tasks such as important sentence
extraction. This algorithm features words which only appear in a target document as more important. We similarly apply the tf -idf algorithm to compute the importance of an evaluative sentence.
First, we divide sentences in reviews to morphemes by using a morphological analyzer1 . Then,
we define the tf -idf value of a word i in target reviews j as below. Note that we treat only content
words (except words such as suffixes and pronouns).
tfji × idfi =Revalllog2 (f req(i, j) + 1)
× {log2 () + 1}
log2 (words(j))
Revinclude(i)(1)

where f req(i, j) is the frequency of i for j, words(j) is the number of words belonging to j,
Revall is the number of all reviews, and Revinclude(i) is the number of reviews including i.
Next, we compute the importance of an evaluative sentence by using the tf -idf value of aword. We denote an evaluative sentence by S = {w1 , w2 , ...}, where each w is a word which has
tf -idf value in the sentence. The importance of an evaluative sentence tf -idfS is as below.
P
tf idf w(2)
tf idfS = w∈S|S|
1

We used Mecab. http://mecab.sourceforge.net/

PACLIC 24 Proceedings

where |S| is the total number of w in the sentence. This is the importance of an sentence based onfeature words.

2.3 The number of mentions with a similar topic
We treat multi-reviews for the target of summarization. In this case, some reviewers might write
similar opinions. These similar opinions have possibilities to be redundantly extracted as a summary. On the other hand, the opinion mentioned by many reviewers is important. We need tohandle redundant information regarding as a feature to determine an important sentence. Therefore, we aim to integrate similar opinions by clustering them.
In this paper, we apply the k-means algorithm which is widely used as the clustering algorithm
because of its simplicity. Since the k-means algorithm is a non-hierarchical method, firstly weneed to specify how many clusters we divide. However, It is difficult to know the optimum numberof divided clusters beforehand. Seki et al. (2007) has estimated the valid number of clusters bystatistically evaluating the clustering result. We apply their algorithm to our task.
We divide evaluative sentences to morphemes and construct a vector space using these morphemes as features. Note that we treat content words, adjectives, and verbs. A score of eachfeature is the tf -idf value computed in section 2.2 if the feature has the tf -idf value. If a featuredoes not have the tf -idf value, we simply assign the frequency within the evaluative sentence as
the score. Besides, we introduce the concept of centrality of the word which has been reported by
Ishii et al. (2001) to characterize features. This method assumes that words such as a subject caseand an objective case in a sentence indicate a topic of the sentence. We also apply the concept to
our task and weight central words in the evaluative sentence. By clustering evaluative sentences
based on these algorithms, we can generate some clusters including similar sentences.
However, we found that clusters generated by our algorithm tended to be divided too much in apreliminary experiment. It denotes that similar opinions which should belong to the same cluster
belong to other clusters.
We revise the clusters by using another approach. In the algorithm mentioned above, eachcluster is formed with evaluative sentences which are gathered by co-occurrence of some words.
Therefore, each cluster contains words which appear in common in the cluster. We identify representative words of each cluster by using this feature. If a cluster contains one sentence, we regard
all nouns and adjectives in the sentence as representative words. In other cases, we regard words
which appear more than half of the number of sentences in the cluster as representative words forthe cluster. Using the representative words, we integrate clusters which contain the same representative words. Since a cluster often contains some representative words, we need to decide whichcluster is the best for integration. In this case, we compare the tf -idf value of representative words
and integrate clusters by using the representative word with the top tf -idf value.

3 Summarizer
In this section, we describe how to generate a summary based on three features mentioned in
section 2. We compute the importance of each cluster by integrating the tf -idf value of eachsentence and the number of mentions. The importance of a cluster Imp(C) is as below.
P
tf idfS× log(|C| + 1)
(3)
Imp(C) = S∈C|C|
where C = {S1 , S2 , ...} is a cluster, each S is an evaluative sentence, |C| is the total numberof sentences in the cluster. The Imp(C) is the importance which means both the importance offeature words and the number of mentions.
Besides, we treat ratings of aspects to reflect the proper balance of whole opinions of reviews.
The process for the sentence extraction is as follows:

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Figure 2: The process of our summary generation.

1. identify a representative sentence which has the top tf -idf from each cluster,
2. classify representative sentences into the rating to which the sentence belongs,
3. extract sentences with high Imp(C) on the basis of the distribution of the total number ofrepresentative sentences belonging to each rating.
Figure 2 shows the process of our summary generation. We generate a summary of target reviewsby arranging extracted sentences.

4 Experiment
In this section, we evaluated our summarization. First, we describe the data set for the experiment.
Then, we evaluate the effectiveness of our clustering method. After that, we generate a summarywith our method. We subjectively evaluate the effectiveness of using the three features. Finally,
we compare our summary with a manual summary for quantitative evaluation.

4.1 Data set
We used game review documents as target data. The review documents were extracted manually from the Web site2 . Seven evaluative criteria are given to each review, i.e., <Originality(o)>, <Graphics (g)>, <Music (m)>, <Addiction (a)>, <Satisfaction (s)>, <Comfort (c)>,and <Difficulty (d)>. We chosen one of the Nintendo DS software “New Super Mario Brothers”
as the target data. There were 170 reviews for this game.
The target data was annotated beforehand. In this experiment, three annotators (A1 , A2 , A3 )annotated the target reviews. They annotated 25 reviews (approximately 450 sentences) in 170 reviews. Annotators detected evaluative sentences from the reviews and annotated aspects to which
the sentences belong. We target the 25 reviews for summarization3 .
For quantitative evaluation, annotators manually generated summaries. Annotators extracted
50 sentences as a summary from the 450 sentences in the 25 reviews. There is no limit how many
sentences they extract for each aspect.

4.2 Evaluation of clusteringAt first, we evaluated the effectiveness of our clustering method. We classified evaluative sentences
detected from reviews into the seven aspects. The clustering was independently-applied in each
aspect. We assigned two sentences with the highest and the lowest tf -idf value as initial clusters.
Table 1 shows the change of the number of clusters in each aspect. “original” is the numberof evaluative sentences (one sentence as one cluster), “k-means” is the number of clusters whichwere applied the original k-means algorithm, and “revised” is the number of clusters which were
integrated by our method. The number of clusters in the revised step decreased to approximately
23

http://ndsmk2.net/
Note that we computed the tf -idf value with 170 reviews.

PACLIC 24 Proceedings

Table 1: The number of clusters in each step.
Annotator

a

c

d

g

m

o

s

A1

original
k-means
revised

50
17
14

83
54
27

49
28
16

26
13
10

18
7
4

124
40
20

90
24
20

A2

original
k-means
revised

43
29
22

73
42
23

67
42
16

33
19
11

19
11
7

160
39
26

115
29
23

A3

original
k-means
revised

46
19
16

83
33
20

51
21
15

27
17
11

15
6
5

54
25
15

198
89
50

1 1. The save system was inconvenient.

2. We should be able to save the data whenever.
3. It’s bad that save point is decided.
4. I can’t save the data in free.

2 1. I can operate Mario without any difficult

technique.
2. “Mario vs. Luigi” stage was very exciting.
3. I played Mario after so long but enjoyed.

1. The operation method is easily
understandable.
2. I can play many actions by easy
operation.
3. I think the method of operation is a
bit strange.

3

Figure 3: An example of integrated clusters after revision.

30 % of the original sentences. We verified the decrease of clusters between the k-means stepand the revised step. In our subjective evaluation, the revised step could form better clusters than
the k-means step. Therefore, we conclude that the revision of the clustering was effective.
Figure 3 shows an example of the clusters in the aspect <Comfort (c)>4 . Each labeled box(1−3) was a cluster. The cluster 1 was a successful example. In this aspect, there were 16 sentences which mentioned a key “save” and 15 sentences were integrated into the same cluster. In
this case, all sentences in this cluster indicated a similar opinion. The “save” was suitable for a
topic of the summarization. On the other hand, in the cluster 2, many sentences were integrated
into the same cluster by co-occurrence of the word “Mario”. We think the reason that “Mario”
tended to frequently appear in all the reviews because of the nature of the product, and it had thehigh tf -idf value. However, “Mario” was not a topic for the summarization. We need to identify words which widely appear and decrease the importance of such words. Besides, the current
clustering method did not handle a polarity of a sentence. As seen in the cluster 3, a cluster might
consist of different polar opinions. We need to deal with the problem; for instance, by using therating information as a feature of the clustering.

4.3 Evaluation of a summary
Using the result of clustering, we generated a summary. For evaluating the effectiveness of eachfeature, we summarized 25 reviews with the following three methods.
Meth1. only the tf -idf value
Meth2. the tf -idf value and Imp(C)Meth3. Meth2 and rating information
Table 2 shows an example of Meth3’s summary for the aspect <Addiction (a)> with A1 ’s
annotation. The “rating” is the actual rating of the review containing the representative sentence, “Ave” is the average of the ratings in the cluster containing the representative sentence,and “ST D” is the standard deviation of the ratings in the cluster.
4

Actually all the sentences in the target data are written in Japanese.

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Table 2: A summary by Meth3 for <Addiction (a)>.
Representative sentence
I can play more if I completed to collect star coins.
The feature of Mario series is that we are not tired.
It feels good to break a stage with big Mario.
I often feel it monotonous play.
I was addicted in collection of coins.
It’s fatal that this game does not have other exciting features.

Table 3: A summary by Meth1

Imp(C)

Rating

Ave

ST D

5.001
3.833
5.994
1.773
8.328
3.445

5
5
4
3
2
0

4.286
4.000
3.600
3.000
3.273
2.000

1.030
0.816
1.020
0.000
1.286
1.633

Table 4: A summary by Meth2

Extracted sentences

Extracted sentences

I was addicted in collection of coins.
Big mushroom item is very funny idea.
Challenge of Star coins collection is good.
It feels good to break a stage with big Mario.
Collection of Star coins is good idea.
I enjoyed collecting Star coins.

I was addicted in collection of coins.
It feels good to break a stage with big Mario.
I can play more if I completed to collect star coins.
I was bored after I cleared this game once.
The feature of Mario series is that we are not tired.
We easily play this game with no stress.

We discuss the effectiveness of Meth3. One of the problems in the result of Table 2 was that
there existed the sentence to which its rating did not correspond. For example, although “I was
addicted to collect coins.” was usually considered as a positive opinion, the rating of the review
containing the sentence was 2. The reason was that most of the sentences in the review containing
the representative sentence had negative opinions for this aspect. This is due to the fact that
opinions of reviewers sometimes might be inconsistent with the ratings. We need to handle thenon-consistence of ratings if we treat the rating information. We verified the difference between
the rating of the representative sentence and the average rating in the cluster. ST D in Table 2 also
indicated that ratings in some clusters varied widely. We think the reason that we do not treat thepolarity for the clustering or opinions in the cluster were not similar because clustering accuracy
was not enough. We need to discuss a more high-accuracy clustering approach and an algorithm
for representative sentence extraction.
We compared the summaries of each method. Summaries by Meth1 and Meth2 are shownin Table 3 and Table 4. As seen in Table 3, we verified that the summarization by Meth1 contained some sentences about “star coin”. It also consisted of only positive opinions. As comparedwith Meth1, Meth3 could extract the proper balance of opinions. It shows that opinion integration and using ratings were effective. On the other hand, there were no great difference between
Meth2 and Meth3. Both methods use the same representative sentences and select them withthe high Imp(C). Therefore, both summarizations became similar. We need to use ratings more
effectively. For instance, by using the rating information for the determination of representative
sentences, Meth3’s result might become better.

4.4 Quantitative evaluation
We compared our summaries with manual summaries. We extracted 50 sentences by using Meth1,
Meth2, and Meth3. We estimated how many sentences we extract for each aspect by using the
distribution of the number of evaluative sentences in each aspect. The estimation approach was
simple idea but we verified the result was relatively close to the manual one.
We used ROUGE-N (Lin, 2004) for evaluation of summaries. It indicates an n-gram recall
between reference summaries and a candidate summary. ROUGE-N is computed as follows:PP
Cm (gN )S∈SHPgN ∈S(4)
ROUGE-N = PS∈SHgN ∈S C(gN )PACLIC 24 Proceedings

Table 5: ROUGE-N between our summaries and manual summaries.
a

c

d

m

g

o

s

Avewgt

N=1

Meth1
Meth2
Meth3

0.287
0.435
0.413

0.496
0.539
0.487

0.347
0.333
0.350

0.421
0.186
0.342

0.082
0.137
0.127

0.363
0.372
0.363

0.383
0.414
0.422

0.345
0.362
0.367

N=2

Meth1
Meth2
Meth3

0.117
0.297
0.223

0.207
0.203
0.146

0.118
0.127
0.130

0.370
0.039
0.287

0.013
0.040
0.030

0.204
0.176
0.178

0.103
0.112
0.115

0.150
0.144
0.151

Table 6: ROUGE-N between annotators.
N=1
N=2

a

c

d

m

g

o

s

Avewgt

0.440
0.232

0.443
0.234

0.402
0.226

0.543
0.337

0.900
0.808

0.459
0.224

0.467
0.184

0.480
0.263

where SH is a set of reference summaries, gN is the N length n-gram, C(gN ) is the frequency of
the gN in the reference summary, and Cm (gN ) is the maximum number of n-grams co-occurringin a candidate summary and a set of reference summaries.
Table 5 shows ROUGE-1 and ROUGE-2 between our summaries and manual summaries.
Those scores are the average scores in three annotators. Since the distribution of each aspect
was different, we computed the weighted average Avewgt . We weighted the number of sentencesin reference summary for each aspect.
Meth3 could achieve the best Avewgt score but there was no significant difference between
three methods. We need to discuss two matters. One is the effectiveness of using the rating information. Meth3 summarized reviews with the rating information but the result showed that it was
not effective. We think the reason that we used rating information only for summary sentence extraction. We can treat the rating information for our clustering method and representative sentence
extraction. We need to improve the use of the rating information.
The other problem is that Imp(C) did not work effectively. We used Imp(C) for Meth2 andMeth3 but the results were not different from Meth1. We think the reason that the representative
sentences were not appropriate. The current method decides a representative sentence of a cluster
by using only tf -idf value of the sentence. The representative sentence might not reflect wholeopinions in the cluster. We need to introduce other features, for instance, a centroid of a cluster, alength of a sentence, and phrase structures.
We computed ROUGE between annotators as reference (Table 6). We regarded A1 as thereference summary and the scores are the average of A2 and A3 . While most of the scores werehigher than the result of Table 5, the scores themselves were not so high. This result shows that itis difficult to generate a same sentiment summary even if between humans5 .

5 Related work
Meng and Wang (2009) have extracted aspects from the specification of the target product andsummarized reviews with hierarchic structures. As a result, they could extract appropriate aspects
for the products. However, the generated summary did not include detailed opinions about the
product. In contrast, our method can treat detailed information by extracting important sentenceswith feature words. Blair-Goldensohn et al. (2008) have computed a polarity value of sentences
based on the maximum entropy method with WordNet and rating information. They extracted
evaluative sentences with a high polarity value preferentially and generated a summary. As theadvantage of their method, it could estimate the polarity of sentences with high accuracy. However,5Note that this result contains the following difficulty; (1) detect same evaluative sentences from reviews, (2) assign
same aspects to evaluative sentences, and (3) select same summary sentences for each aspect.

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it is not always true that sentences with high polarity values are appropriate for a summary. They
also did not treat redundancy of the summary. Lu and Zhai (2008) have introduced the concept of
aspects to a PLSA model. They integrated expert reviews and ordinary opinions scattering in theWeb. Opinions which should be integrated are identified by measuring the number of mentions in
the Web. Although their method are very effective, their purpose is to add more information, suchas similar and supplementary opinions, to the base review.
In this paper, we did not discuss the identification of the aspect of sentences. However, it isimportant to identify the aspect information for the sentiment summarization task. Hadano et al.
(2010) have identified an aspect of an evaluative sentence with machine-learning approaches. We
need to introduce such a method to our task in the future.

6 Conclusion
In this paper, we focused on the multi-aspects review summarization. We handled three features;
ratings of aspects, the tf -idf value, and the number of mentions with a similar topic. We usedthe clustering method to integrate similar opinions. The experiment result showed that we could
integrate similar opinions and it led to the redundancy elimination of a summary. We evaluatedthe effectiveness of using all three features for summarization. In our subjective evaluation, we
verified that the summarization by using the three features was effective. However, in quantitativeevaluation, there was no great difference between the summary using all three features and summaries using only one or two features. Future work includes (1) improvement of the clustering
algorithm, (2) more appropriate decision of representative sentences, and (3) treating the rating
information more effectively.

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