Thesaurus Construction and Analysis Method for Dialogue Understanding<BR>
--Thesaurus Construction for Dialogue Understanding--
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Hiroaki TSURUMARU, Hideyuki MAEDA, and AKihiro KAWASHIMA
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Department of Electrical Engineering and Computer Science, 
Nagasaki University<BR>
1-14 Bunkyou-machi, Nagasaki 852, JAPAN<BR>
e-mail: turumaru@ec.nagasaki-u.ac.jp
<P>  
    Many ellipses and demonstrative pronouns occur in dialogue. 
Generally speaking, the omitted words (or phrases) and the pronominal 
references are complemented by the use of common sense and discourse 
information. Here it becomes a serious problem for the dialogue 
understanding that the definition of common sense is not clear. 
The thesaurus consisting of the semantic (hierarchical) relations such 
as upper/lower relation or part/whole relation between 
words is regarded as an approximate model of the common sense. 
There are some thesauri such as ``Bunrui-Goi-Hyo (Word List by Semantic 
Principles)'' and ``Roget's Thesaurus''. However, they are not 
always sufficient for natural language processing, because they
are mainly for the use of human beings.
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    This study aims to clarify the method for constructing a thesaurus 
based on hierarchical relations such as upper/lower relation 
and part/whole relation between the concepts of words, and to approach 
to the problems of the application of the thesaurus to dialogue 
understanding. Here we regard one of the senses of a word as one 
concept. Now, how and from what to obtain these hierarchical
relations is one of the most important problems for constructing the 
thesaurus.
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   We have been studying how to acquire these hierarchical relations 
from the definition sentences in the on-line Japanese dictionary, 
and developing a programming system for computer-aided thesaurus 
construction. The contents of the current year's studies are mainly as 
follows: First, we review the algorithm for extracting the hierarchical 
relations. Second, we discuss the evaluation of the trial thesaurus 
which has been made on an experimental basis through the results of 
these works. And third, we also discuss the application of the thesaurus
to presumption of the elliptical words in dialogue.
Now we describe those three topics more specifically.
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    (1) Concerning the extracting algorithm, we have reviewed it 
from a theoretical viewpoint. The basic idea of the extraction
of the hierarchical relations is as follows; generally the definition 
sentence contains the core word(s) expressing the central meaning of 
the word sense, which we call the definition word(s). Then we extract 
the definition word(s) and the relational information, and decide the 
semantical relation between the entry word and the definition word. 
Here the semantical relations include, as well as upper/lower
relation and part/whole relation, synonymous relation and element/set 
relation. We also regard the latter two relations as hierarchical 
relation in a wide sense.
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<BR>
    (2) Concerning the evaluation of the trial thesaurus, we have 
researched on the followings;
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<P>
<LI>	[a] the number of superordinates (upper words) and the number of 
	hyponyms (lower words) contained in each superordinate
<LI>	[b] the definition of the depth, the number of passes in each 
	depth and the average depth on the conceptual hierarchy
<LI>  	[c] the utilization of the by-pass to infer the valid pass 
   	in the multi-passes
<LI>  	[d] the assignment of the superordinates to the local maximal 
   	words
<LI>  	[e] the logical extension of the words-pairs satisfying 
   	part/whole relation with the aid of upper/lower	relation
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<BR>
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    (3) Concerning the application of the thesaurus, we have studied 
algorithm for the inference of the omitted words or phrases in 
dialogue using the thesaurus and IPAL basic verbs dictionary in order 
to verify the validity of the trial thesaurus. The outline of the 
algorithm is as follows;
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<LI> 	morphological analysis of input sentence,
<LI>  	estimation of the deep structure (case structure), 
<LI>	recognition of the existence of the omitted words or phrases,
<LI>	presumption of the omitted words or phrases.
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<BR>
   This algorithm can be used to handle pronominal and 
anaphoric reference.
We have collected the dialogue data for experiments from the texts of 
NHK Sequel Basic English(1991).  
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Keywords : thesaurus, semantic dictionary, word knowledge, conceptual 
	   hierarchy