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

6 Chapter 1. Introduction
by the conceptual base, they establish a smaller synonymy network. This network
is finally exploited for the identification of word clusters, which can be
seen as new synsets.
3. Ontologisation: the lexical items in the arguments of the non-synonymy relation
instances are attached to suitable synsets. Once again, this is achieved
by exploiting the network established by all extracted relations, in order to,
given a relation instance, select the most similar pair of candidate synsets.
As the resulting resource is structured in synsets and semantic relations between
them, it can be seen as a wordnet. Given the three aforementioned steps, this
approach for creating wordnets automatically was baptised as ECO, which stands
for Extraction, Clustering and Ontologisation.
1.3 Contributions
Given our main goal, Onto.PT can be seen as the main contribution of this research.
Onto.PT is a wordnet-like lexical ontology for Portuguese, whose current
version integrates lexical-semantic knowledge from five lexical resources, more precisely
three dictionaries and two thesauri. Actually, after noticing that most of
the Portuguese lexical resources were somehow complementary (Santos et al., 2010;
Teixeira et al., 2010), we integrated in Onto.PT those that were public.
The current version of Onto.PT contains more than 100,000 synsets and more
than 170,000 labelled connections, which represent semantic relations. This new
resource is a public alternative to existing Portuguese LKBs and can be used as a
wordnet. This means that, for Portuguese, Onto.PT can be used in most NLP tasks
that exploit the structure of a wordnet for achieving their goal, except for those that
use the synset glosses, unavailable in Onto.PT.
But Onto.PT is not a static resource. It is created in a three step flexible
approach, ECO, briefly described in the previous section. ECO enables the integration
of lexical-semantic knowledge from different heterogeneous sources, and
can be used to create different instances of the resource, using different parameters.
Moreover, although applied only to the creation of Onto.PT, we propose ECO as an
approach that may be adopted in the creation or enrichment of wordnets in other
languages. It is thus another important contribution of this thesis.
Each step of ECO can also be individually seen as contribution to the fields
of information extraction and automatic creation of wordnets. These steps include
procedures for:
1. Enriching an existing thesaurus with new synonymys.
2. Discovering synsets (or fuzzy synsets) from dictionary definitions.
3. Moving from term-based to synset-based semantic relations, without accessing
the extraction context.
On the other hand, the procedure for extracting semantic relations from dictionaries
cannot be seen as novel. Still, we have compared the structure and contents
in different dictionaries of Portuguese, which led to the conclusion that many regularities
are kept across the definitions of each dictionary. This comparison,