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

108 Chapter 6. Thesaurus Enrichment
most ambiguous word (Max(senses)). On the synsets (table 6.9), we present their
quantity (Total), their average size in terms of words (Avg(size)), the number of
synsets of size 2 (size = 2) and size greater than 25 (size > 25) and, also, the size
of the largest synset (max(size)).
Thesaurus POS
TeP 2.0
1 st iteration
2 nd iteration
Clusters
TRIP
Words
Total Ambiguous Avg(senses) Max(senses)
Noun 17,149 5,802 1.71 20
Verb 8,280 4,680 2.69 50
Adjective 14,568 3,730 1.46 19
Adverb 1,095 227 1.30 11
Noun 28,693 11,794 1.98 22
Verb 11,272 6,357 2.85 50
Adjective 19,148 7,149 1.85 21
Adverb 1,865 499 1.40 12
Noun 29,223 11,988 1.99 22
Verb 11,301 6,374 2.86 50
Adjective 19,291 7,213 1.85 21
Adverb 1,914 513 1.40 12
Noun 21,126 2,196 1.14 5
Verb 1,801 177 1.13 4
Adjective 4,687 359 1.10 5
Adverb 743 89 1.15 3
Noun 45,457 15,392 1.80 22
Verb 11,924 6,607 2.87 52
Adjective 22,316 7,782 1.83 22
Adverb 2,488 694 1.42 12
Table 6.8: Thesauri comparison in terms of words.
After the assignments, the number of words grows and the number of synsets
becomes slightly lower. This might seem strange, but as some synsets in TeP are
very similar to each other, after the assignments, they become the same synset, and
one of them is discarded. Furthermore, as expected, ambiguity becomes higher at
this stage. As there is the same number of synsets, but more words, some words
are added to more than one synset. And the synsets also become larger, as they are
augmented.
The thesaurus obtained after clustering is smaller and much less ambiguous than
the others. Besides the high threshold (θ = 0.5), this happens because the words
not covered by TeP tend to be less frequent, which are typically more specific and
thus less ambiguous. Nevertheless, for nouns, there is still a synset with 31 words.
The words in TRIP are slightly more ambiguous than in TeP and the synsets of
TRIP are also larger than TeP’s. It is clear that TRIP is much larger than TeP. It
contains about two and a half times more noun and adverb lexical items, about 3,500
more verbs and 8,000 more adjectives. The highest number of synsets means that
the new thesaurus is broader also in terms of covered natural language concepts.
On the other hand, the new thesaurus is more ambiguous and has larger synsets.
For instance, it has almost 600 synsets with more than 25 words, which can be seen
as too large for being practical (Borin and Forsberg, 2010). TeP has just 66 of those
synsets. Nevertheless, we have looked to the largest synsets of TRIP and noticed
that most of them are well-formed as they only contain synonymous words.