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

4 Chapter 1. Introduction
too much human effort, which may be seen as a bottleneck for the development of
the resource. Not to mention that, over time, language evolves with its users.
The truth is that as long as there is intensive labour involved in manually encoding
lexical resources, lexical capabilities of NLP systems will be weak (Briscoe,
1991), and coverage limitations will always be present. The same happens for other
kinds of knowledge base – handcrafting them is impractical and undesirable. We
should therefore take advantage of available NLP tools in order to automate part of
this task and reduce the need of manual input (Brewster and Wilks, 2004).
Having this in mind, especially before the establishment of WordNet, researchers
studied how to automatise the task of acquiring lexical-semantic knowledge from
text, with relative success. For instance, MindNet (Richardson et al., 1998) shown
that it is possible to develop a lexical(-semantic) knowledge base (LKB) by automatic
means.
Another common alternative to the manual creation of wordnets is the translation
of a target wordnet (usually Princeton WordNet) to other languages (de Melo
and Weikum, 2008). However, another problem arises because different languages
represent different socio-cultural realities, they do not cover exactly the same part
of the lexicon and, even where they seem to be common, several concepts are lexicalised
differently (Hirst, 2004). Therefore, we believe that a wordnet for a language,
whether created manually, semi-automatically or automatically, should be developed
from scratch for that language.
As mentioned before, the manual creation of a knowledge base results in slow
development and consequently in limited coverage, not only of lexical, but mostly
on world knowledge. This is why, after the establishment of WordNet, researchers
using this resource as their only knowledge base soon had to cope with information
sparsity issues. So, apart from the work on the automatic construction of LKBs from
scratch, there have been automatic attempts to enrich wordnets (e.g. Hearst (1998))
and also to link them with other knowledge bases (e.g. Gurevych et al. (2012) or
Hoffart et al. (2011)), in order to create broader resources.
In the work described in this thesis, we look at the Portuguese scenario, and
tackle the limitations of the LKBs for this language. We developed an automatic
approach for the acquisition of lexical-semantic information and for the creation
of lexical-semantic computational resources, dubbed ECO – Extraction, Clustering,
Ontolgisation. The application of ECO to Portuguese resources resulted in a
wordnet-like resource, dubbed Onto.PT. In the remaining of this chapter, we state
the main goals of this research, briefly present the ECO approach, refer the main
contributions of this work, and describe the structure of the rest of this thesis.
1.1 Research Goals
For the Portuguese language, there have been some attempts to create a wordnet
or a related resource (see more in section 3.1.2), but all of them have one or more
of the following main limitations:
• They are proprietary and unavailble, or their utilisation is not free;
• They are handcrafted, and thus suffer from limited coverage;