A Wordnet from the Ground Up

108 Chapter 4. Extracting Relation InstancesMetaphor is a major source of errors and, even more so, are relations betweenlarger noun phrases, which the patterns assign only to the heads. A typical situation:NomToNom captures NLU2 that includes a relative clause, but only the head is considered.Even a nominal modifier in genitive or an adjectival modifier often makes themeaning of the noun phrase different from the lexical meaning of the head. The conditionsin mIInne do not constrain the case of the nominal LUs, so it is quite commonto erroneously recognize hyponymy for a noun in genitive that is not the head. It isnot easy, however, to identify complex Polish noun phrases in genitive. The error ratewould be cut if we could apply a good chunker or even a shallow parser combinedwith the analysis of the meaning relations between structurally related noun phrases –see, for example, (Jacquemin, 2001).Examples of LU pairs extracted by all three patterns appear in Figure 4.3. Figure 4.4presents examples of LU pairs extracted from the joint corpus by each of the threepatterns.The results of the application of lexico-morphosyntactic patterns are valuable, butthere remains an impression that more could be achieved by following the main line ofthe pattern-based paradigm. We will now shift our attention to approaches to automaticextraction and evaluation of more generic patterns.4.3 Generic Patterns Verified StatisticallyA manual construction of lexico-syntactic patterns is not laborious if we rely more onintuition than on an intensive survey of known hypernymy instances and the context oftheir occurrences in a corpus. Morin and Jacquemin (1999) proposed semi-automateddiscovery of lexico-syntactic patterns. Given a predefined list of hypernymy instances,sentences including these LU pairs are extracted and transformed into “lexico-syntacticexpressions”. Next, common environments that generalise the expression are producedby considering the similarity of the expressions and a generalisation procedure: lexicosyntacticpatterns describing commonalities of expression subgroups are deduced. Thepattern extraction procedure still assumes manual verification of the deduced patterns,and the patterns are next applied without automatic evaluation of their accuracy and thereliability of the extracted pairs. The latter is especially important for the applicationof generic (weakly constraining) patterns to large corpora.Manually constructed patterns are claimed to have good precision but very lowrecall (Hearst, 1998). Recall can be increased by using more generic patterns extractedautomatically from a corpus, with broad coverage but intrinsically low precision.Most of the proposed methods follow the common scheme: given the initial examplesof the target relations, henceforth called seeds, patterns are generated from thecorpus and next used to extract further instances. Methods differ in pattern generation