Journal of Intelligent Learning Systems and Applications, 2011, 3, 131-138
doi:10.4236/jilsa.2011.33015 Published Online August 2011 (
Copyright © 2011 SciRes. JILSA
Insertion of Ontological Knowledge to Improve
Automatic Summarization Extraction Methods
Jésus Antonio Motta, Laurence Capus, Nicole Tourigny
Département D’Informatique et de Génie Logiciel, Université Laval, Québec, Canada.
Email:, {laurence.capus, nicole.tourigny}
Received May 30th, 2011; revised July 20th, 2011; accepted July 27th, 2011.
The vast availability of information sources has created a need for research on automatic summarization. Current
methods perform either by extraction or abstraction. The extraction methods are interesting, because they are robust
and independent of the language used. An extractive summary is obtained by selecting sentences of the original source
based on information content. This selection can be automated using a classification function induced by a machine
learning algorithm . This function classifies sentences into two groups : important or non-importan t. The important sen-
tences then form the summary. But, the efficiency of this function directly depends on the used training set to induce it.
This paper proposes an original way of optimizing this training set by inserting lexemes obtained from ontological
knowledge bases. The training set optimized is reinforced by ontological knowledge. An experiment with four machine
learning algorithm s was made to valida te this proposition. The improvement achieved is clearly significant for each of
these algorithms.
Keywords: Automatic Summarization, Ontology , Machine Learning, Extraction Method
1. Introduction
Research works on automatic summarization have greatly
increased in recent years. Indeed, digital sources of in-
formation have become increasingly available. When a
user runs a query on Internet, she/he must choose among
the retrieved documents those containing relevant infor-
mation for her/him. The task becomes more difficult
when the number of documents increase. An automated
system able to ‘discover’ the essential informatio n is one
of the challenges of artificial intelligence, especially in
natural language processing. In some cases, methods of
machine learning based on symbols are used to tackle
this problem.
Automatic summarization can be seen as a problem of
transforming one or more documents in a shorter version
with preserving information content [1]. The methods
used are divided into two main approaches: extraction
and abstraction, respectively surface methods and deep
methods in a more linguistic viewpoint. A summary ob-
tained by extraction is composed of a set of sentences
selected from the source document(s) by using statistical
or heuristic methods based on information entropy of
sentences. The summarization process by extraction is a
relevant alternative, robust and independent of language,
compared with the summarization process by abstraction
[2]. An abstractive summary is obtained by semantic
analysis in order to interpret the source text, and find
new concepts to generate a new text that will be the
summary. This method requires linguistic processing at a
certain level [3]. In addition, a summary can be produ ced
in a generic way to give a general idea of the contents of
documents to be summarized. It can also be based on
keywords supplied by the user. In this case, it will con-
tain the most relevant information related to these key-
words [4]. Automatic summarization process by abstrac-
tion is usually decomposed into three steps: interpreta-
tion of source document(s) to obtain representation,
transformation of this representation, and production of a
textual synthesis [5]. Both approaches have their advan-
tages and drawbacks. For this research, we are only in-
terested in automatic summarization process by extrac-
tion and how to improv e it.
The main problem of this kind automatic summariza-
tion by extraction lies in identifying the most important
information of the document sources [2]. Different
methods have been used until now with more or less
successful results according to measurements based on
recall (the number of correct sentences selected on the
Insertion of Ontological Knowledge to Improve Automatic Summarization Extraction Methods
total number of correct sentences) and precision (the
number of correct sentences on the total number of se-
lected sentences) [6]. Some methods use an ontology or
ontological knowledge to analyze terms and relations [7].
More recently, other methods have been reported using
machine learning algorithms for determining descrip-
tions of concepts. These methods build a training set di-
vided into two subsets: important sentences and non-
important sentences [1]. This training set is next used to
induce a classification function from the concepts de-
scription. This function will serve to classify future sen-
tences to produce new summaries. Generally in classifi-
cation problems, the set of attributes is very large and
entropic, with much noise and irrelevant attributes. The
well-known underlying problem is named the curse of
dimensionality [8]. Indeed, data too scattered do not fa-
cilitate a good estimate, nor obtain good classification
models. This problem is actually tackled by using heuris-
tic methods based on linear approximations, which opti-
mize the training set by reducing it or constructing a new
smaller set from another series of attributes [9]. The ob-
tained results so far, even if they have progressed, could
be further improved.
In this paper, we propose to optimize the training set
in an original way. We insert lexemes of ontological
knowledge bases into the training set to form a concept-
tual space, which will be used by the learning algorithm.
Our hypothesis is that is possible to obtain a reinforced
set, by using ontological knowledge to select or trans-
form the characteristics of the set. We validated our hy-
pothesis with four machine learning algorithms. We
compared their performance by using various evaluation
indicators. The obtained results showed that our solution
improves the performance; it is then promising for the
suite of this research. In Section 2, we will describe the
solution proposed. In Section 3, we will presen t the con-
ducted experimentation to validate our solution. In Sec-
tion 4, we will conclude our paper by giving future work.
2. Insert Ontological Knowledge in
Summary Extraction Process
Automatic summarization by extraction is a broad topic
that uses different approaches, methods or techniques. It
seems important at first to give our research framework,
i.e. the process that we have considered and decided to
improve. Then, we explain what we mean by the insertion
of ontological knowledge and how this insertion fits into
the summarization process. Finally, we give the evalua-
tion methods that have allowed us to validate our hypo-
2.1. Summarization Process Considered
The different methods used for automatic summarization
by extraction can be grouped into three approaches: sta-
tistical, enriched statistical and machine learning [5]. In
this research work, we are interested especially in ma-
chine learning approaches because the results obtained
are relevant and promising. The key item of these ap-
proaches lies on the choice of the training set and its op-
timization, which will be used to induce the classification
function for summarizing futures documents in function
of information content. More precisely, the sentences of
the documents are represented by vectors, which consti-
tute an initial matrix [10 -12]. This matrix corresponds to
the training set. The induced classification function en-
ables to classify sentences into two classes: class 1 for
important sentences and class –1 for non-important sen-
tences. The summary will be then composed of the sen-
tences of class 1. The crucial problem of this process is
the fact that the sentences of this matrix are very entropic.
It is necessary to optimize the matrix in order that it be-
comes an efficient training set.
Although many efforts have been made to improve the
quality of summaries obtained, thus approaching those
achieved by humans, there are still gaps in terms of ac-
curacy and precision of results. Moreover, most of the
summaries obtained are built from a single document.
The most evident explanation is that the problems of
redundancy increases along with the number of docu-
ments to be summarized.
The idea of our research work is then to propose a so-
lution to better optimize the training set, i.e. the set of
selected sentences forming the initial matrix needed for
inducing the classification function. We wanted to find a
solution more efficient, which do not need initially sum-
maries already written to constitute the training set and
can be applied on several documents to be summarized.
2.2. Insertion of Ontological Knowledge
Before inserting ontological knowledge, we identify the
sentences of the document(s) to be summarized. Next,
we apply a syntactic analysis and delete stop words. We
then create a matrix E, formed of words by sentences.
Each item of the matrix contains the value tf × idf of the
word i in the sentence j. This discriminatory value is
based on the Salton et al.’s fo rmula [13], which evaluates
the value of a term compared to a corpus of documents.
We insert ontological knowledge to this matrix in order
to obtain the new matrix E0. Our hypothesis lies on the
fact that the training set is reinforced by new information,
i.e. terms or items with more semantic content and po-
tentially discriminatory. Su ch an insertion also enab les to
solve partially the problem of synonymy [1], one of main
open problems of information retrieval. Briefly, the ini-
tial matrix is improved by adding a set of sub-trees of
hypernym and hyponym, for each word.
opyright © 2011 SciRes. JILSA
Insertion of Ontological Knowledge to Improve Automatic Summarization Extraction Methods133
We elaborated an algorithm to insert ontological know-
ledge that enables to find conceptual structures from on-
tology, to compute their importance in an information
viewpoint and introduce th em in each term of the matrix.
This algorithm gives lexemes in function of the words of
sentences and inserts their semantic values to the matrix.
The optimum lexemes have a factor that adds a semantic
value to items of the matrix, improving its performance
for classification. More precisely, the algorithm begins to
do a search in the ontology by subject and verb. Next, it
identifies the various concepts of each sentence by ana-
lyzing different sub-trees of parts of the sentence. The
sub-trees are built in function of semantic relations of
hypernym and hyponym. The algorithm evaluates the
various sub-trees and chooses the best one. To finish, it
inserts the selected sub-tree in the training set. When all
new components of the training sets are inserted we have
a new conceptual space enriched.
After inserting ontolog ical knowledge, we do different
steps to obtain the final training set and then induce the
classification function. First, we filter entropic attributes
with algebraic methods. To obtain our new set, we used a
similarity transformation matrix, which enables to find
smaller and less redundant subsets of attributes. By ap-
plying a transformation matrix to the matrices E and E0,
we id enti fy prin cipa l compo nents [8] and singular values
[14] in order to reduce the entropy of the matrix and sort
sentences in function their information content. The
principal components of a matrix enable to identify
groups of variables/words (principal component), greatly
connected in the group, but without correlation between
groups. The determining factor of this grouping is the
variability, which represents the information or impor-
tance. We can then choose the first sentences, with the
greatest variability, as important sentences and the last
ones as non-important.
In detail, we represent the matrix E (for singular val-
ues for instance), by :
0 L 0
M 0 MM
0 L s
UVss s
 
Each value si corresponds to the variability of each
sentence in all sentences. This v ariability is correlated to
the information content of each sentence. We associate
the values si, the greatest to the smallest, with the corre-
sponding words of the matrix. Next, we label the sen-
tences that contain high values si as important (class 1)
and those that contain low values si as non-important
(class –1). Finally, we use machine learning algo rithm to
induce the classification function, among those proposed
by literature.
The induced classification function is ab le to differenti-
ate sentences with much information (important) to sen-
tences with insufficient information (non-important). So
by applying the induced function on new documents,
only the sentences containing the most important infor-
mation are chosen to produce summaries.
2.3. Evaluation Method
To evaluate our solution and verify its efficacy, we cre-
ated a contingency table, named confusion matrix. The
inputs of this table correspond to considered classes,
knowing that these values are given after applying the
classification function to the training set. As the training
set is a binary set, we obtain Table 1 elaborated in func-
tion of the two classes to be determined: important and
When the process of classification is finished, we
identify four categories of sentences among all those
TP: if the function predicts correctly a sentence la-
beled as important;
TN: if the function predicts correctly a sentence la-
beled as non-important;
FP: if the function predicts incorrectly a sentence la-
beled as important;
FN: if the function predicts incorrectly a sentence la-
beled as non-important.
We used the information given by Table 1 to obtain
the values of three evaluation indicators known in auto-
matic summarization, that are recall, precision and F-
score, as well as ROC curves (Receiver Operation Char-
acteristic) [15].
Recall (R) is the number of predictions TP divided by
the true number of positive instances classified as posi-
tives [6]. It informs about the capability of the classifica-
tion function to identify a sentence as important when it
is really important. The following formula enables to
compute recall: TP
Precision (P) corresponds to the number of predictions
TP divided by the total number of instances classified as
Table 1. Confusion matrix used to evaluate efficacy.
Predicted Class
Class Important Non-important
Important True Positive Case (TP) False Negative Case
Non-important False Positive Case
(FP) True Negative Case (TN)
Copyright © 2011 SciRes. JILSA
Insertion of Ontological Knowledge to Improve Automatic Summarization Extraction Methods
Copyright © 2011 SciRes. JILSA
positives [6]. This indicator informs about the capability
of the function to classify correctly a sentence according
to all the sentences added to this category. It is computed
with the following formula:
Fscore corresponds to a harmonic average of recall and
precision [6] and it is defined by:
We also represented our results by using ROC curves.
These graphs enable to represent all the pairs of values
TPR (True Positive Rate) or sensitivity and FPR (False
Positive Rate) or 1-specificity, resulting of a continuous
variation of the observation points in the whole row of
observed results [15]. By simple observation of these
graphs, we obtain a qualitative comparison. When we
apply each model to be evaluated on the training set, the
curve placed on the top and to the left has the greatest
accuracy. Likewise, the area under the curve indicates
the success probability of the model by identifying a
sentence as important. The ROC curves then give indica-
tions on the accuracy of the classificatio n model, as well
as a unified criterion in the evaluation process. The men-
tioned values are obtained by the following formulas:
sensitivy(TPR)TP FN
specificity(TNR)1 FPR
Recall, Precision and Fscore as well as ROC curves al-
lowed us to evaluate the improvement rates obtained by
our solution
3. Experiment and Results
The experiment is conducted on a set of documents from
Reuters Corpus [16], a news database that contains ap-
proximately 11000 documents, classified into 90 currents
events subjects and grouped into two sets, respectively
named training and test. Each document contains on av-
erage 120 words and 15 sentences. We chose a total
number of 2000 documents for our exp eriment.
For the extraction of ontological knowledge, we used
WordNet database [17], developed by Princeton univer-
sity. This is a database oriented semantically with a very
rich frame, greatly used in computational linguistic. It is
composed of words related to names, verbs, adjectives
and adverbs. Words are organized into sets of synonyms
named synsets, related by semantic relations of hy-
pernym, hyponym, meronymy and holonomy. WordNet
database thus contains 155287 words and 117659 syn-
We also chose four machine learning algorithms
greatly used. The first algorithm is named Support Vec-
tor Machine [18]. It builds a hyper-plane in an n-dimen-
sions space to classification, regression or other tasks.
Intuitively, a good separation between classes is obtained
when a hyper-plane has the greatest distance for all the
nearest points of the training set. The second algorith m is
a probabilistic classifier based on Bayes’ theorem, Baye-
sian Classifier or Naïve Bayes [9], but with a great inde-
pendence hypothesis. In other words, it assumes that the
presence or absence of a characteristic is not related to
the presence or absence of another characteristic. The
third algorithm, Random Tree [19], realizes its classifica-
tion by building a tree in which the total number of se-
lected nodes is randomly chosen while being equal to:
log (attribute number1)
Finally, the fourth algorithm is Multilayer Perceptron
[20]. This is an artificial neuronal network with many
layers. The activation function of each neuron is not lin-
ear. This neuronal network can be used to identify line-
arly inseparable classes. The function is learned from
multilayers that are totally connected to each other.
The experiment conducted is composed of two parts.
In the first part, we produced summaries by extraction
from the chosen corpus without inserting ontological
knowledge to the machine learning algorithm. W e evalu-
ated the obtained results for each algorithm in function of
the evaluation methods given. In the second part, we
produced summaries from the same corpus, this way by
inserting ontological knowledge. We also evaluated the
results obtained. By following, we present these results
and discuss them.
3.1. Results for Recall, Precision and Fscore
We used methods of random subsampling (1/3 for the
test set and 2/3 for the training set) and cross-validation
(10 crossings). We also based on Tanagra software of
Lyon University [21], Weka software of Waikato Uni-
versity [22] and Orange software of Ljubljana University
[23]. The two methods used gave similar results.
Tables 2 and 3 present the values of recall, precision
and Fscore as well as the confusion matrix for each of the
four algorithms evaluated. In Table 2, the algorithms
were applied to the training set obtained from principal
omponents of the word matrix, while, in Table 3, the c
Insertion of Ontological Knowledge to Improve Automatic Summarization Extraction Methods135
Table 2. Predictions and confusion matr ix wi th pr incipal components.
Algorithm Without ontological knowledge With ontological knowledge
Prediction Confusion matrixPrediction Confusion matrix
Recall Precision Fscore Important Non-
Important Recall PrecisionFscore Important Non-
Naïve Bayes 0.993 0.978
Important 1.00 0.986 488 0 0.9940.964 480 18
Non-important 0.837 1.00 7 36 0.8180.964 18 81
1.00 0.999
Important 1.00 1.00 488 0 1.000.998 483 0
Non-important 1.00 1.00 0 43 0.9991.00 1 98
Random Tree 0.982 0.9521
Important 1.00 0.972 488 0 0.9880.919 472 6
Non-important 0.674 1.00 14 29 0.5790.905 42 57
ML Per c ept ron 0.993 0.983
Important 0.998 0.988 487 1 0.9920.974 479 4
Non-important 0.861 0.974 6 37 0.8690.956 13 86
Table 3. Predictions and confusion matrix with singular values.
Algorithm Without ontological knowledge With ontological knowledge
Prediction Confusion matrixPrediction Confusion matrix
RecallPrecision Fscore ImportantNon-
important Recall PrecisionFscore Important Non-
Naïve Bayes 0.997 0.958
Important 1.00 0.994 486 0 0.9980.956 483 1
Non-important 0.858 1.00 3 18 0.776.987 22 76
1.00 0.999
Important 1.00 1.00 486 0 1.00 0.998 484 0
Non-important 1.00 1.00 0 21 1.0 1.00 1 97
Random Tree 0.992 0.957
Important 1.00 0.984 486 0 1.0 0.917 484 0
Non-important 0.61 1.00 8 13 0.5511.0 44 54
ML Per c ept ron 0.992 0.980
Important 1.00 0.984 486 0 0.9710.999 470 14
Non-important 0.619 1.00 8 13 0.9490.869 5 93
training set was obtained from singular values. The val-
ues are given for each part of the experiment, i.e. before
inserting ont ological knowle d ge and after inserting it.
By observing Tables 2 and 3, we note that perform-
ances in terms of recall and precision are high. Naïve
Bayes algorithm presents the greatest performance in the
case of using principal components. Its performance is
followed by the two other algorithms Support Vector
Machine and MultiLayer Perceptron. When ontological
knowledge is inserted then the greatest performance is
those of Support Vector Machine algorithm followed by
those of MultiLayer Perceptron algorithm. In the case
where the space was based on singular values, this is
Support Vector Machine algorithm that obtains the grea-
test performance, followed by those of Naïve Bayes al-
gorithm. When the ontological knowledge is inserted,
Copyright © 2011 SciRes. JILSA
Insertion of Ontological Knowledge to Improve Automatic Summarization Extraction Methods
Support Vector Machine algorithm continues to occupy
the first place. The performance of MultiLayer Percep-
tron algorithm is improved at the expense of those of
Naïve Bayes algorithm.
3.2. Results for ROC Curves
In Table 4, we observe the AUC values (Area Under
Curve) of each ROC curves, for each algorithm applied
at a training set obtained from principal components. The
results are given before and after inserting ontological
knowledge. The last column indicates the relative im-
provement obtained.
Table 5 gives the same values, but this way by con-
sidering that the training set is obtained from singular
From the joint observation of Tables 4 and 5, we can
say that the introductio n of ontological knowledge in the
training set, obtained using principal components or sin-
gular values, increases the quality of all algorithms. For
instance, we note an improvement of 58.8% and 19.1%
for respectively MultiLayer Perceptron and Random
Tree algorithms when using principal components. Sup-
port Vector Machine and Naïve Bayes algorithms are
improved of 14.8 % and 10.33 % respectiv ely, when using
singular values. The algorithm with the best performance
is Support Vector Machine followed by Naïve Bayes,
when using principal components. In the case of singular
values are used, we also observe a very great improve-
ment of the algorithm quality when the ontological
knowledge is inserted. MultiLayer Perceptron algorithm
obtains again the highest value with 27%. Support Vector
Table 4. Improvement when using principal components.
Values of ROC curves
Algorithm Without onto-
logical knowl-
With ontological
Improvement (%)
Naïve Bayes 0.668 0.737 10.33
SVM 0.682 0.783 14.8
Random Tree 0.555 0.661 19.1
ML Perceptron 0.449 0.713 58.8
Table 5. Improvement when using singular values.
Values of ROC curves
Algorithm Without onto-
logical knowl-
With ontological
Improvement (%)
Naïve Bayes 0.683 0.820 20.06
SVM 0.673 0.811 20.51
Random Tree 0.697 0.740 6.17
ML Perceptron 0.589 0.748 26.99
Machine and Naïve Bayes algorithms occupy respect-
tively the second and the third places with 20.5% and
20.1%. The best quality algorithm is Naïve Bayes fol-
lowed by Support Vector Machine.
Table 6 shows a comparison of AUC values of each
algorithm after inserting ontological knowledge, de-
pending on whether the training sets are obtained from
principal components or singular values.
Table 6 enables to estimate the difference of im-
provement between a space based on principal compo-
nents and another based on singular values. First, we
observe that all algorithms stud ied improve their qualita-
tive performance moving from a space of principal com-
ponents to another of singular values. The greatest im-
provement is obtained by Random Tree algorithm with
12% followed by Naïve Bayes with 11.3%. From this
table, we also conclude that the two best algorithms and
the most promising to extract summaries with spaces
ontologically reinforced, among the four ones studied,
are Naïve Bayes and Support Vector Machine.
Figures 1 and 2 correspond to ROC curves obtained
for each algorithm with training sets produced from
principal components and singular values. In these two
cases, the results are given without inserting ontological
knowledge (1a), and with inserting ontological knowl-
edge (1b).
As we already mentioned, the ROC curves offer a way
of evaluating the quality of a classification algorithm in
function of its capability to give good prediction s. In our
case, a good prediction corresponds to sentences classi-
fied as important, that should take part of summary, and
discriminated to non-important sentences.
In addition to the information obtained by Ta bles 4-6,
careful observation of figures enables to identify the op-
timum points of each algorithm, simply by placing the
point at the h ighest position to the left. We also compare
the accuracy between algorithms. For instance, if we
want to compare Support Vector Machine algorithm
when it reaches a little more than 90% of TP cases face
to Ran dom Tree algo rithm then we see on the Figure 2(b)
that Support Vector Machine algorithm has an appro-
Table 6. Difference of improvement between using princi-
pal components and using singular values.
Values of ROC curves
Algorithm Principal
mponentcos Singular
Naïve Bayes 0.737 0.820 11.30
SVM 0.783 0.811 3.60
Random Tree 0.697 0.740 12.00
ML Perceptron 0.589 0.748 4.90
opyright © 2011 SciRes. JILSA
Insertion of Ontological Knowledge to Improve Automatic Summarization Extraction Methods137
Figure 1. (a) Principal components before inserting onto-
logical knowledge; (b) Principal components after inserting
ontological knowledge.
ximate FP rate of 37% and Random Tree algorithm has a
rate of 70%.
The obtained results show a significantly improvement
to the classification function after inserting ontological
knowledge, whatever the machine learning algorithm
used. More, these results give the two best algorithms.
The Naïve Bayes and Support Vector Machine algo-
rithms should be then applied to automatic summariza-
tion with a training set produced from singular values
and reinforced by ontological knowledge.
4. Conclusions
There are still great opportunities for deepening and de-
velopment research to find suitable methods for summa-
rizing. In this paper, we studied the behaviour of four
machine learning algorithms that induce classification
Figure 2. (a) Singular values before inserting ontological
knowledge; (b) Singular values after inserting ontological
function from training sets. These sets were reinforced
by inserting ontological knowledge and used to dis-
criminate the important sentences, from one or several
documents, of those which are not. The used algorithms
are Naïve Bayes, Support Vector Machine, Random Tree
and Multilayer Perceptron. By analyzing the results of
experimentation, we concluded that all considered algo-
rithms may be used to produce summaries. We also note
that using principal components or singular values to
select the training set may be successfully retained to
induce the learning functions of the four studied algo-
rithms. The insertion of ontological knowledge gives
qualitative improvements of performance remarkable.
This insertion enables to propose good classification
functions, which are able to discriminate sentences be-
Copyright © 2011 SciRes. JILSA
Insertion of Ontological Knowledge to Improve Automatic Summarization Extraction Methods
Copyright © 2011 SciRes. JILSA
tween important and non-important. The sentences dis-
criminated as important constitute the future summary.
Likewise, we observe that ontological knowledge pro-
duces more great effects on the classifier quality, if the
training set is obtained from singular values rather than
from principal components. From this final analysis, we
conclude that the two best algorithms that should be ap-
plied to auto matic summarization by extraction are Naïve
Bayes and Support Vector Machine, from a set of singu-
lar values reinforced by ontological kn o wl e d ge.
As future work, we think that it would be interesting to
evaluate the performance of classification algorithms on
more reduced spaces, i.e. optimized by means of tech-
niques different to those used in this experimentation. It
would be also interesting to explore their behaviour on
sets reinforced by ontological knowledge.
5. Acknowledgements
The authors would thank Natural Sciences and Engi-
neering Research Council of Canada (NSERC) for its
financial support.
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