Onto.PT: Towards the Automatic Construction of a Lexical Ontology for Portuguese

214 Pages·2013·2.13 MB·English

PhD Thesis
Doctoral Program in Information Science and Technology
Artificial Intelligence
Onto.PT: Towards the Automatic Construction of
a Lexical Ontology for Portuguese
Hugo Gon¸calo Oliveira
Thesis Advisor
Paulo Jorge Sousa Gomes
Department of Informatics Engineering
Faculty of Sciences and Technology
University of Coimbra
September 2012

This thesis was submitted, on 21st September 2012, to the University of Coimbra,
for the fulfillment of the requirements for obtaining the degree of Doctor of
Philosophy in Science and Information Technology.
The argument of this thesis took place on 27th May 2013, in Sala dos Capelos,
University of Coimbra. The candidate was unanimously approved with distinction
and honors by the following jury:
• Fernando Am´ılcar Bandeira Cardoso, Full Professor at Faculdade de Ciˆencias
e Tecnologia, University of Coimbra, Portugal (Chair)
• Roberto Navigli, Associate Professor at the Department of Computer Science,
Sapienza University of Rome, Italy
• Diana Santos, Professor at Faculty of Humanities, University of Oslo, Norway
• Maria Lu´ısa Torres Ribeiro Marques da Silva Coheur, Assistant Professor at
Instituto Superior T´ecnico, Technical University of Lisbon, Portugal
• Ana Alexandra Ribeiro Lu´ıs, Assistant Professor at the Faculdade de Letras,
University of Coimbra, Portugal
• Paulo Jorge Sousa Gomes, Assistant Professor at Faculdade de Ciˆencias e
Tecnologia, University of Coimbra, Portugal

Acknowledgements
Although I am the only one signing this thesis, the completion of the work de-
scribed here would have not been possible without many people, who helped me
both scientifically and personally.
I would like to start by thanking my supervisor, Paulo Gomes, who was there
every week for listening to me, for giving me his expert opinion on each detail of
this work, and for encouraging me to go further.
The Master’s works of Let´ıcia Anton P´erez and Hernani Costa were key to the
development of this work, so I would like to give them a special thanks for their
effort, for the long and very interesting discussions, and for believing in my ideas.
I would like to thank everybody in the KIS laboratory, because they were some-
how important for the development of this work. Special remarks to Ricardo Ro-
drigues, with whom I worked together on P´agico and who developed the lemmatiser
and the API for the POS-tagger used in this work; also to Bruno Antunes, for
keeping everything running at the lab.
I would like to thank Alberto Simo˜es, for the support and interactions about
Diciona´rio Aberto; and Rui Correia and Nuno Mamede for providing me the list of
cloze questions used to evaluate Onto.PT indirectly.
I would also like to thank all the reviewers involved in the manual evaluation of
one or more parts of this work and who have not yet been mention, namely, Lu´ısa
Ponte, PauloSantos, AnaOliveiraAlves, FilipeRodrigues, MarisaFigueiredo, Jorge
´
Santos, Diogo Silva, Raul Sa´ and Joel Cordeiro. Also Jorge Avila, for proofreading
this thesis.
My PhD work was financially supported by the FCT scholarship grant with
the reference SFRH/BD/44955/2008, co-funded by FSE. But I would like to thank
CISUC as well, for hosting me as their PhD student, and for providing me most
services a PhD student needs, including the financial support for attending scientific
events and present my work.
I would like to give a special word to Linguateca, and especially to Diana Santos,
who always believed in me, even in the beginning, when my experience in NLP was
limited. Without all the knowledge and enriching experience I acquired through
Linguateca, I am absolutely sure that I would have never felt ready for a PhD.
More personally, this work would have not been possible without both the full
support of Clara and of my mother, Margarida. They both take care of me so well,
that they make it easier, even on those days when everything seems to be going
the wrong way. To all my family, including those that did not make it to see me
completing this work. And to everybody that I met along the last 5 years because,
in some way, they have all contributed to setting my mood and were an additional
source of inspiration for this work.

Preface
Aboutsixyearsago,almostbyaccident,Iendedupengaginginanacademicresearch
career. It all started with my Master’s dissertation, the final, and probably the most
important, stage of my Master’s degree. Then, I was not planning to dedicate more
than one year of my life to research. But even one year later, when I started working
as a researcher for Linguateca, it was far from my thoughts that I would soon enroll
on a PhD.
Briefly, the main goal of my Master’s work was to, given a rhythmic sequence,
generate matching lyrics, in Portuguese. My intention was always to work with my
mother tongue – not only because I felt that the results would be more understand-
able and funnier for the people surrounding me, but also because I used to write a
few Portuguese lyrics for my former band. I was thus very interested in investigating
how far an automatic lyricist could go.
However, working with Portuguese revealed to be a challenging task. Since the
beginning of the work, we noticed that there was a lack of language resources for
Portuguese and it was not easy to find the few existing ones. For instance, at
that time, we could not find a public comprehensive lexicon for providing words
and information on their morphology and possible inflections. Not to mention a
semantics-oriented lexicon. Since then, I decided I wanted to contribute with some-
thing useful, that would hopefully fulfill the aforementioned shortage of resources.
More or less at the same time, I had my first contact with Linguateca, a distributed
language resource centre for Portuguese, responsible not only for cataloguing exist-
ing resources, but also for developing and providing free access to them.
I was very lucky that, before the end of my Master’s, Linguateca opened a po-
sition that I applied for. The main goal of this position was to develop PAPEL,
a lexical-semantic resource for Portuguese, automatically extracted from a dictio-
nary. After my Master’s, I was hired for that precise task. While working for
Linguateca, I started to have a deeper contact with other researchers working on
the computational processing of Portuguese. I started to gain some experience on
natural language processing (NLP), especially on semantic information extraction,
and I became passionate for research in this area. So much that, today, I do not see
myself doing something completely unrelated.
The work with Linguateca was very important for my training as a researcher
in NLP. It was so enriching that I felt that, with what I had learned, I could do,
and learn, more. And there is so much to do to contribute to the development of
Portuguese NLP, that I wanted to continue my work, which I did, after embarking
on my PhD. This thesis presents the result of a four year PhD where, starting with
what we learned with PAPEL, we created a larger resource, Onto.PT, by exploiting
othersources,andwedevelopedamodelfororganisingthisresourceinanalternative
way, which might suit better concept-oriented NLP.

Abstract
The existence of a broad-coverage lexical-semantic knowledge base has a positive
impact on the computational processing of its target language. This is the case
of Princeton WordNet, for English, which has been used in a wide range of nat-
ural language processing (NLP) tasks. WordNet is, however, created manually by
experts. So, despite ensuring highly reliable contents, its creation is expensive, time-
consuming and has negative consequences on the resource coverage and growth.
For Portuguese, there are several lexical-semantic knowledge bases, but none
of them is as successful as WordNet is for English. Moreover, all of them have
limitations,thatgofromnothandlingambiguityatthewordlevelandhavinglimited
coverage (e.g. only nouns, or synonymy relations) to availability restrictions.
Having this in mind, we have set the final goal of this research to the automatic
construction of Onto.PT, a lexical ontology for Portuguese, structured in a similar
fashiontoWordNet. Onto.PTcontainssynsets–groupsofsynonymouswordswhich
are lexicalisations of a concept – and semantic relations, held between synsets. For
this purpose, we took advantage of information extraction techniques and focused
on the development of computational tools for the acquisition and organisation of
lexical-semantic knowledge from text.
Our work starts by exploring textual sources for the extraction of relations,
connecting lexical items according to their possible senses. Dictionaries were our
first choice, because they are structured in words and meanings, and cover a large
part of the lexicon. But, as natural language is ambiguous, a lexical item, identified
by its orthographical form, is sometimes not enough to denote a concept. Therefore,
in a second step, we use a synset-based thesaurus for Portuguese as a starting point.
The synsets of this thesaurus are augmented with new synonyms acquired in the
first step, and new synsets are discovered from the remaining synonymy relations,
after the identification of word clusters. In the last step, the whole set of extracted
relations is exploited for attaching the arguments of the non-synonymy relations to
the most suitable synsets available.
In this thesis, we describe each of the aforementioned steps and present the
results they produce for Portuguese, together with their evaluation. Each step is a
contributiontotheautomaticcreationandenrichmentoflexical-semanticknowledge
bases, and results in a new resource, namely: a lexical network; a fuzzy and a
simple thesaurus; and Onto.PT, a wordnet-like lexical ontology. An overview of the
current version of Onto.PT is also provided, together with some scenarios where it
may be useful. This resource, which can be further augmented, is freely available for
download and can be used in a wide range of NLP tasks for Portuguese, as WordNet
is for English. Despite the current limitations of an automatic creation approach,
we believe that Onto.PT will contribute for advancing the state-of-the-art of the
computational processing of Portuguese.