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

Chapter 6
Thesaurus Enrichment
General language dictionaries and language thesauri cover the same kind of knowledge,
but represent it differently. While the former consist of lists of word senses
and respective natural language sense descriptions, the latter group synonymous
words together, so that they can be seen as possible lexicalisations of concepts.
WordNet (Fellbaum, 1998) can actually be seen as a resource that bridges the gap
between both kinds of resources, because each synset contains a textual gloss.
However, in previous chapters, we have shown that, even though they intend to
cover the same kind of knowledge, most of the information in public handcrafted
Portuguese thesaurus is complementary to the information extracted from dictionaries.
Therefore, it should be more fruitful to integrate their information in Onto.PT
instead of using them merely as a reference for comparison. Another aspect in favour
of this option is that, besides its size, TeP was manually created by experts. This
means that, more than integrating the information in TeP, we can take advantage
of its structure to have more reliable synsets and more controlled sense granularity.
The work presented in this chapter can be seen both as an alternative or a
complement of the previous chapter, as we use the synsets of TeP as a starting
point for the construction of a broader thesaurus. To this end, we follow a fourstep
approach for enriching an existing electronic thesaurus, structured in synsets,
with information extracted from electronic dictionaries, represented as synonymy
pairs (synpairs) 1 :
1. Extraction of synpairs from dictionary definitions;
2. Assignment of synpairs to suitable synsets of the thesaurus;
3. Discovery of new synsets after clustering the remaining synpairs;
4. Integration of the new synsets in the thesaurus.
In step 1, any approach for the automatic acquisition of synpairs from dictionaries,
such as the one described in chapter 4, may be followed. Therefore, we will not
go further on this step. We start this chapter by presenting its main contribution,
which is the algorithm for the automatic assignment of synpairs to synsets. Then,
we evaluate the algorithm against a gold standard and select the most adequate
settings for using it in the enrichment of TeP. Any graph clustering procedure suits
step 3 of our approach. We chose to follow an approach similar to the one introduced
1 Synpairs are synonymy tb-triples. They can be extracted from several sources, however, as we
are dealing with general language knowledge, dictionaries are the obvious targets.