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

7.2. Ontologising performance 119
Finally, the pair of synsets (Ai, Bj), such that Ai and Bj maximise P R(Ai) and
P R(Bj) respectively, is selected.
Minimum Distance (MD): This algorithm assumes that related synsets contain
terms that are close in N. For this purpose, it selects the closest pair of synsets,
given the average (edge-based) distance of their terms:
dist(Ai, Bj) =
|Ai| |Bj|
dist(aik, bjl)
k=1 l=1
|Ai||Bj|
(7.7)
The minimum distance between two nodes is the number of edges in the shortest
path between them, computed using Dijkstra’s algorithm (Dijkstra, 1959). If a
term in a synset (aik or bjl) is not in N, they are removed from Ai and Bj before
this calculation. If this algorithm was applied for attaching ontologise the tb-triple
{a R1 b}, given the situation of figure 7.1, there would be several ties for the best
pair of synsets, because this network is simpler than most real networks.
7.2 Ontologising performance
For Portuguese, TeP is the only freely available lexical resource with synset-relations.
However, these relations are limited to antonymy, which is not a very prototypical
semantic relation. Therefore, in order to quantify the performance of the algorithms
presented in the previous section, and to compare them for ontologising different relations,
we have created a gold reference, manually, with a set of tb-triples extracted
from dictionaries and their plausible attachments to the synsets of two handcrafted
thesauri. Only after this, we used TeP as a gold resource and the algorithms for
ontologising antonymy tb-triples.
This section starts by describing the resources involved in the creation of our
handcrafted gold reference and reports on the results using each algorithm for ontologising
hypernymy, part-of and purpose-of tb-triples. Then, we present the results
of ontologising antonymy tb-triples in TeP.
7.2.1 Gold reference
The gold reference for this evaluation consisted of the synsets of TeP 2.0 and
OpenThesaurus.PT, where samples of tb-triples from PAPEL 2.0 were attached.
Synsets
In order to eliminate the noise from automatic procedures, we decided to include
only synsets from handcrafted thesauri in our gold reference. As referred in the
previous chapters, for Portuguese, there are currently two free handcrafted broadcoverage
thesauri: TeP 2.0 (Maziero et al., 2008) and OpenThesarus.PT (OT.PT).
TeP is the largest by far (see section 3.1.2) and is created by experts, so its synsets
were the best alternative for our gold reference. However, TeP was created for