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

154 Chapter 8. Onto.PT: a lexical ontology for Portuguese
1. POS-tag the original sentence, with the correct answer in the blank, so that it
is coherent. After tagging, remove the answer from the sentence.
2. Run the Personalized PageRank WSD algorithm (see section 8.4.2) using all
content words of the sentence as context. This means that the initial weights
are uniformly distributed to all synsets with, at least, one context word. The
rest of the nodes have initial weights = 0.
3. For each alternative answer, retrieve all candidate synsets.
4. Select the alternative in the highest ranked synset.
The first algorithm was run for answering the 3,900 questions with the help of
CARTÃO, and also using, independently, the lexical networks that this resource
merges – those extracted from DLP (PAPEL), Dicionário Aberto (DA), and Wiktionary.PT.
The second algorithm was used for answering the questions with the
help of Onto.PT. Table 8.8 shows the accuracy values obtained.
Resource Nodes Accuracy
CARTÃO lexical items 41.8%
PAPEL 3.0 lexical items 39.8%
DA lexical items 36.6%
Wiktionary.PT lexical items 35.8%
Onto.PT synsets 37.6%
Table 8.8: Accuracy on answering cloze questions.
These results are clearly higher than the random selection, which is 25%, because
there are four possibilities for each question. We should add that our approach is
not very sophisticated, and takes advantage only of the question’s context and of
the LKBs. But there are a few questions with a short context – 355 have less than 8
context words – as well as several questions with named entities – 1,752 contain at
least one capitalised word – which are not expected to be found in a LKB. Figure 8.7
is an example of a question with a named entity (with three tokens) and context
of five words, which was not answered correctly using any of the resources. On the
other hand, figure 8.8 shows a question that was answered correctly using each of
the resources.
Mercedes Classe C carácter desportivo.
(a) reforça (c) desloca
(b) defende (d) implica
Figure 8.7: Cloze question not answered correctly, using any of the resources.
Despite the poor accuracy of the results obtained, this exercise showed that the
organisation of the used LKBs makes sense, and that these resources can be exploited