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

162 Chapter 9. Final discussion
Anyway, before growing in terms of new relations and exploited resources,
Onto.PT will probably shrink, as we will try to minimise some problems, including
incorrect extractions, that currently add noise to its contents. As it is generated by
an automatic approach, and although it is very large, broad-coverage, and shown to
be useful, Onto.PT is still far from being highly reliable. Therefore, some directions
should be taken in order to improve its quality.
The manual evaluation of the extracted semantic relations is an important source
for identifying specific problems, which might lead to future changes in the extraction
grammars or in the filters applied after extraction. We can also exploit other
sources of information, in order to compute the confidence of the extracted semantic
relations. A common approach for this task relies on the application of similarity
measures, based on the the occurrences of the relation arguments in corpora, or in
the Web (Downey et al., 2005; Costa et al., 2011). These approaches could yet be
combined with other kinds of information, including the frequency of the extracted
relation, the confidence on the resource or method that lead to its extraction 2 , and,
as referred earlier, the occurrence of a relation in some resource in other language,
after its translation. In a similar fashion to Wandmacher et al. (2007)’s work, this
could lead to an integrated confidence.
Besides other benefits, a confidence value would enable the integration of only
relations for which confidence is above a predefined threshold. This threshold could
be an additional parameter to study in an extensive evaluation of ECO and Onto.PT.
Such an evaluation would compare the impact of different parameters, including,
but not limited to, the clustering thresholds and the similarity measures used in
ontologisation. The idea would be to select different parameters to create different
versions of the resource and then compare properties like covered lexical items and
semantic relations, sense granularity or synset size. Another parameter could be
a threshold on the corpus frequency of the integrated lexical items. Since much
of the covered knowledge is extracted from dictionaries, Onto.PT contains some
unfrequent, and possibly less useful, words. Besides being of no use for several
applications, those words might as well work as additional sources of noise.
One final mention should be given to the adoption of the new spelling reform
of Portuguese, agreed by the governments of the Portuguese speaking countries in
1990, but only started to be implemented in 2009. This reform aims to unify the
orthography of the European and Brazilian variants of Portuguese. In Onto.PT,
however, we have not adopted this reform because:
• All resources we have exploited are not yet converted. As some of them are
written in the European variant of Portuguese, others in the Brazilian, and
others in both, most of the written variations are covered, as well as some
dropped forms;
• The transition period, where using dropped written forms is tolerated, is still
going on in most of the countries. In Portugal, it ends in 2015.
• There is still a huge debate going on the adoption of this reform, and on its
real benefits for Portuguese;
Nevertheless, we believe that an eventual conversion of Onto.PT to the new spelling
2 Given that a thesaurus as TeP is created manually, there is certainly more confidence on a
relation acquired directly from it than one extracted from text, by an automatic procedure.