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Top1. Introduction
With the advancement of modern information technology, we are facing the increasing need of language processing technologies in Eastern languages like Chinese. During past centuries, ontology building becomes an important part in semantic-level linguistic and knowledge processing, and meanwhile, since most of our knowledge is incarnated within free text as the form of natural language, development of linguistic processing is helping to better ontology learning. Hyponymy relations play an important role in knowledge engineering and the acquisition of which becomes an essential and crucial problem. The hierarchy structure of hyponymy relations composes the skeleton of knowledge bases and application of which ranges natural language processing, information retrieval, machine translation to other related domains.
Several knowledge sources are used in hyponymy acquisition, three primary types of which are: structured corpus (De Meo, Terracina, Quattrone & Ursino, 2004), semi-structured corpus (Dolan, Vanderwende & Richardson, 1993) and unstructured corpus (Cao & Shi, 2001). The largest among the three is unstructured text, the research of which has attracted a lot of researchers and has become a key research area. Thanks to recent research effort on knowledge engineering, new knowledge sources, such as large scale Chinese concept set extracted from unstructured corpus (Wang, Cao, Cao and Cao, 2007; Zhou, Wang & Cao, 2007), are available and have provided rich information.
There are three mainstream approaches—the Symbolic approach, the Statistical approach and the Hierarchical approach—to discovery general hyponymy relations automatically or semi automatically (Du & Li, 2006). The Symbolic approach, depending on lexicon-syntactic patterns, is currently the most popular technique (Hearst, 1992; Liu, Cao, Wang & Chen, 2006; Liu, Cao & Wang, 2005; Ando, Sekine & Ishizaki, 2003). Hearst (1992) was one of the early researchers to extract hyponymy relations from Grolier’s Encyclopedia by matching 4 given lexicon-syntactic patterns, and more importantly, she discussed about extracting lexicon-syntactic patterns by existing hyponymy relations. Liu (2005, 2006) used the “isa” pattern to extract Chinese hyponymy relations from unstructured Web corpus, and have been proven to have a promising performance. Zhang (2007) proposed a method to automatically extract hyponymy from Chinese domain-specific free text by three symbolic learning methods. The statistical approach usually adopts clustering and associative rules. Zelenko, Aone and Richardella (2003) introduced an application of kernel methods to extract two certain kinds of hyponymy relations with promising results, combining Support Vector Machine and Voted Perception learning algorithms. The hierarchical approach is trying to build a hierarchical structure of hyponymy relations. Caraballo (1999) built a hypernymy hierarchy of nouns via a bottom-up hierarchical clustering technique, which was akin to manually constructed hierarchy in WordNet.