Similarity between Words Computed by Spreading Activation on an English Dictionary

Similarity between Words Computed by Spreading Activation on an English Dictionary Hideki Kozima Course in Computer Science .... and Information Mathematics, Graduate School, University of Electro-Communications 1-5-1, Chofugaoka, Chofu, Tokyo 182, Japan (xkozima@phaeton. cs. uec. ac. j p) Teiji Furugori Department of Computer Science and Information Mathematics, University of Electro-Communications 1-5-1, Chofugaoka, Chofu, Tokyo 182, Japan Tel. +81-424-83-2161 (ex.4461) (furugori@phaet on. cs. uec. ac. jp) Abstract This paper proposes a method for measur- ing semantic similarity between words as a new tool for text analysis. The simi- larity is measured on a semantic network constructed systematically from a subset of the English dictionary, LDOCE (Long- man Dictionary of Contemporary English). Spreading activation on the network can di- rectly compute the similarity between any two words in the Longman Defining Vocab- ulary, and indirectly the similarity of all the other words in LDOCE. The similarity rep- resents the strength of lexical cohesion or semantic relation, and also provides valu- able information about similarity and co- herence of texts. 1 Introduction A text is not just a sequence of words, but it also has coherent structure. The meaning of each word in a text depends on the structure of the text. Recogniz- ing the structure of text is an essential task in text understanding.[Grosz and Sidner, 1986] One of the valuable indicators of the structure of text is lexical cohesion.[Halliday and Hasan, 1976] Lexical cohesion is the relationship between words, classified as follows: 1. Reiteration: Molly likes cats. She keeps a cat. 2. Semantic relation: a. Desmond saw a cat. It was Molly's pet. b. Molly goes to the north. Not east. c. Desmond goes to a theatre. He likes films. Reiteration of words is easy to capture by morpho- logical analysis. Semantic relation between words, which is the focus of this paper, is hard to recognize by computers. We consider lexical cohesion as semantic similarity between words. Similarity is Computed by spread- ing activation (or association) [Waltz and Pollack, 1985] on a semantic network constructed systemati- cally from an English dictionary. Whereas it is edited by some lexicographers, a dictionary is a set of asso- ciative relation shared by the people in a linguistic community. The similarity between words is a mapping a: Lx L ---* [0, 1], where L is a set of words (or lexicon). The following examples suggest the feature of the similarity: a(cat, pet) = 0.133722 (similar), a(cat, mat) = 0.002692 (dissimilar). The value of a(w, w') increases with strength of se- mantic relation between w and w'. The following section examines related work in or- der to clarify the nature of the semantic similarity. Section 3 describes how the semantic network is sys- tematically constructed from the English dictionary. Section 4 explains how to measure the similarity by spreading activation on the semantic network. Sec- tion 5 shows applications of the similarity measure -- computing similarity between texts, and measuring coherence of a text. Section 6 discusses the theoret- ical aspects of the similarity. 2 Related …

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