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

146 Chapter 8. Onto.PT: a lexical ontology for Portuguese
acto, ...). Another important difference regards the size and the correction of the
synsets of MWN.PT and Onto.PT. The former contains small synsets, often with
only one word, while the latter, as referred earlier, contains large synsets. On the
other hand, given its manual revision, the MWN.PT synsets are supposedly all
correct, while Onto.PT, due to its automatic construction, contains incorrections.
8.4 Using Onto.PT
The main goal of creating Onto.PT is its exploitation in the achievement of tasks
on the computational processing of Portuguese. As referred earlier in this chapter,
this is also a popular approach to validate ontologies (Brank et al., 2005).
In order to illustrate the utility of a resource as Onto.PT, in this section, we
provide utilisation scenarios, where this resource can be seen as a valuable contribution.
All the scenarios intend to be mere proofs of concept. None of the used
techniques are very sophisticated and we did not go further on their evaluation. We
start by presenting an exercise on exploring the taxonomy of Onto.PT. Then, we
show how Onto.PT can be applied to word sense disambiguation (WSD). After that,
we briefly describe how this resource was integrated in an information retrieval (IR)
system, in order to enhance query expansion. The IR system was evaluated with
the participation in an IR joint task. The last utilisation scenario is about taking
advantage of Onto.PT to answer cloze questions automatically.
8.4.1 Exploring the Onto.PT taxonomy
The first usage example is a simple exploration exercise, showing that, besides providing
synonyms for lexical items, Onto.PT can be queried to acquire taxonomic information,
as well as other semantic information on the organisation of the lexicon.
Figure 8.5 shows a four-level taxonomy obtained from Onto.PT, where cão (dog)
is included. For the sake of simplicity, we omit some synset entries, as well as
non-hypernym relations, from this figure.
The taxonomy shows that Onto.PT can be used, for instance, to collect a list of
animals, a list of mammals, or a list of dog breeds. Starting with the most general
level, with a synset denoting an animal, its is possible to obtain kinds of animals,
including birds (ave), insects (insecto), and mammals (mamífero). Mammals can
be expanded for obtaining mammal synsets, including cow (vaca), whale (baleia),
cat (gato), or dog (cão). Finally, if the hypernyms of the dog synset are expanded,
several dog breeds are shown, including boxer, mongrel (rafeiro) or dalmatian
(dálmata).
8.4.2 Word sense disambiguation
There is a wide range of knowledge-based WSD algorithms, using a wordnet both
as sense inventory and as an additional source of knowledge (e.g. Resnik (1995);
Banerjee and Pedersen (2002); Agirre and Soroa (2009)). As Onto.PT is structured
in a similar fashion to a wordnet, most of the previous algorithms may be adapted to
use Onto.PT for performing Portuguese WSD. We have implemented two algorithms
for this task: Bag-of-Words and Personalized PageRank (Agirre and Soroa, 2009).