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

7.2. Ontologising performance 121
• Only tb-triples supported by CETEMPúblico (Santos and Rocha, 2001), a
newspaper corpus of Portuguese, were used. This was done based on the
results of the automatic validation, as reported in section 4.2.5. We thus had
some confidence on the quality of the triples, as their arguments co-occurred
at least once in the corpus, connected by discriminating textual patterns for
their relation.
• Triples with the following frequent but abstract arguments were discarded:
acto (act), efeito (effect), acção (action), estado (state), coisa (thing), qualidade
(quality) as well as tb-triples with arguments with less than 25 occurrences
in CETEMPúblico. Some of the frequent and abstract arguments were
actually considered as “empty heads” (see more on section 3.2.1 of this thesis)
since PAPEL 3.0. This means that, in the current version of PAPEL, there
are not hypernymy tb-triples where these words are the hypernym.
Furthermore, we unified all meronymy relations (part-of, member-of, containedin,
material-of) in a unique type, part-of. This option relied on the fact that the
distinction of different meronymy subtypes is sometimes too fine-grained, and because,
as it occurs for English (Ittoo and Bouma, 2010), for Portuguese there are
textual patterns that might be used to denote more that one subtype.
Attachments
From the previous selection of tb-triples, we chose those held between words included
in, at least, one TePOT synset, and whose attachment raised no doubts. It was
possible to have tb-triples where all possible attachments were correct, as well as
tb-triples without a plausible attachment, because the sense of one of the arguments
was not covered by the thesaurus.
For each tb-triple, the gold reference contained all plausible attachments, as in
the examples of figure 7.5. In the end, the gold reference consisted of 452 tb-triples
and their possible attachments, with those that were plausible marked. Table 7.1
shows the distribution of tb-triples per relation type, the average number of possible
attachments, and the average number of plausible attachments. The proportion of
plausible attachments per tb-triple can be seen as the random chance of selecting a
plausible attachment from the possible alternatives. This number is between 40%,
for hypernymy, and 50% for purpose-of.
Relation tb-triples
Attachments
Avg(possible) Avg(plausible)
Hypernym-of 210 13.7 5.5 (40.2%)
Part-of 175 11.2 5.5 (49.5%)
Purpose-of 67 13.5 6.8 (50.1%)
Table 7.1: Matching possibilities in the gold resource.
7.2.2 Performance comparison
In order to compare the performance of the algorithms, we used them to ontologise
the 452 tb-triples in the gold reference into the candidate synsets. However, instead