A Wordnet from the Ground Up

118 Chapter 4. Extracting Relation Instancesjest = be number=sg,person=3rd , hypo = hyponym, hiper = hypernym, subst = substantive,nom and inst are case values, nominative and instrumental.Pantel and Pennacchiotti (2006) write that patterns can be induced by any patternlearningalgorithm, but only the longest common substring algorithm proposed byRavichandran and Hovy (2002) was used. The same algorithm was the basis for thegeneralisation and unification of patterns in Estratto. The algorithm is heuristicallyguided by a predefined list of relation-specific LUs. Hypernymy, for example, can besignalled by być (be), stać się (become), taki (such), inny (other), and so on.In Espresso, the inferred patterns are then generalized by replacing all multiwordterms (subsets of noun phrases) by the TR labels. Such for Polish might be unworkable:a robust definition of a multiword term as a regular expression seems unattainable – notto mention the lack of a chunker for Polish. As a slightly different method, matchinglocations are specified via morphological similarity to contexts (partial morphologicalspecification: part of speech and values for the selected grammatical categories), andvia predefined relation-specific LUs.The instance extraction phase follows patterns induction and selection. An instanceis a pair 〈x, y〉 of LUs – instances of the target semantic relation. The authorsof Espresso suggest that, given a small corpus, two methods can be used to enrich theinstance set. First, each multiword LU in an instance can be progressively simplifieddown to the head, for example, new record of a criminal conviction → new record →record. A new instance is created the simplified first LU and the second LU intact.Second, a pattern is instantiated only with either x or y, and new instances are retrievedfrom the Web or an additional large corpus. For example, given the pair (dog, animal)and the Estratto pattern(hypo:subst:nom) is a/an (hyper:subst:inst)we create two queries:dogis a/an (hyper:subst:inst)(hypo:subst:nom) is a/an animalInstances collected using both these methods are added to the instance set. Let usnote that in all experiments described by Pantel and Pennacchiotti (2006) only onewordLUs have been used, and the corpora were presumably large enough to providestatistical evidence.Generalized patterns we described are not considered generic so long as they do notgenerate ten times more instances than the average number of instances extracted byreliable patterns from the previous iteration. High recall, however, results in decreasedprecision, so every instance extracted by a generic pattern is verified. The verificationprocess starts with instantiating all non-generic patterns with the instances to be