A Wordnet from the Ground Up

116 Chapter 4. Extracting Relation Instancesof small frequencies, even after discounting dependent on the number of occurrences)can cause lower assessment of patterns with a balanced ratio of co-occurrence withmatched instances versus the pattern occurrences and instance occurrences alone. Suchsituations result in artificially increased values of max pmi . We would like to look fora measure which would be less sensitive to the low frequency of pattern matches orinstances matched. Also, the value 1 of pattern reliability is not guaranteed even fora pattern which occurs only with a subset of seeds, because of the max pmi value whichcan be increased by some infrequent pattern. That is why the propagation of reliabilityto the subsequent iterations causes new values (calculated for patterns from instancesand vice versa) to become gradually lower for the respective set. We seek a measureof reliability which returns 1 as the value for the best patterns or instances in everyiteration.r π (p) =∑i∈I (pmi(i, p) ∗ r t(i)) ∗ d(I, p)max p(∑i∈I (pmi(i, p) ∗ r t(i)) ) ∗ |I|(4.6)d(i, p) defines how many unique instances the given pattern is associated with.PMI in formula (4.6) is usually also modified by a discounting factor.The proposed modifications are intended to increase the reliability of the patterns,which not only extract a lot of instances, but occur with a large number of differentinstances. The modified measure, when applied to instances, promote those whichoccur often in the corpus associated with many different patterns.The choice of the pattern structure is crucial for their expressiveness and the abilityto capture those elements of the language structures that express semantic relationsbetween LUs (such as the linear order of constituents in English), but case-marking ofnoun phrases in Polish (their linear order is mostly insignificant for the potential lexicalsemantic relation between them). Espresso follows roughly the scheme proposed byHearst (1992): patterns are regular expressions, in which the alphabet includes wordforms and the label TR for any multiword term, and a set of variables for noun phrasesmatched as elements of an instance. The role of part-of-speech tags is unclear in theapproach of Pantel and Pennacchiotti (2006), but they are present in the example ofthe generalisation of a sentence [p. 115]:Because/IN TR is/VBZ a/DT TR and/CC x is/VBZ a/DT y.We assumed that patterns are simplified regular expressions, with the Kleene closurebut without grouping. The alphabet for an inflectional language like Polish shouldrather include roots than (numerous) word forms. Espresso patterns rely to some extenton the positional, linear syntactic structure of an English sentence. Porting toa significantly different language may be problematic.