A Wordnet from the Ground Up

Chapter 4Extracting Instancesof Semantic RelationsPattern-based approaches originate from the observation that dictionary definitionsin Machine-Readable Dictionaries [MRD] follow a limited number of fixed schemescharacterised also by a limited range of syntactic constructions. A typical dictionarydefinition of a word sense describes it by a genus term followed by set of differentiaeor a short description referring to related words, e.g. (Matsumoto, 2003). Work on theextraction of lexico-semantic relations from MRD started already in the 1980s (Amsler,1981), following (Matsumoto, 2003). Dictionary definitions are processed usingpatterns that identifying selected expressions. Patterns either are regular expressionsor or are written in a formal language of a similar expressive power.In corpora, patterns are used to recognise pairs of LUs as instances of a specifiedlexico-semantic relation. Consider, for example, a pattern from the seminal work ofHearst (1992):NP 0 ... such as NP 1 , NP 2 ... (and | or ) NP nIt implies that each noun phrase NP i is a hyponym of the noun phrase NP 0 , or,more precisely, the hypernymy relation holds between LUs represented in the text bythe given noun phrases. Hearst (1992, 1998) constructed manually only five patternsfrequently matched in a corpus and appealingly accurate. The accuracy was measuredas a number of LU pairs linked by the hyponymy relation in PWN to all those extracted.For the pattern shown above, for example, 61 of 106 extracted LU pairs from GrolierEncyclopedia were confirmed in PWN (Hearst, 1992).The implicit assumption here is that one can construct patterns accurate enoughto draw correct conclusions from single occurrences of pairs of LUs. In general ,however, it seems barely possible due, amongst others, to the presence of metaphor.Without deeper semantic and pragmatic analysis, instances of metaphor may be hardto distinguish from literal uses. Hearst extracted aeroplane as a hyponym of targetand Washington as an instance of nationalist; such derived associations are clearlyspecific to particular documents from which they were extracted. Another problem isthe scarcity of pattern instances in corpora; merely 46 instances were acquired from20 million words of the New York Times corpus (Hearst, 1992).101