A Wordnet from the Ground Up

5.3. Lessons Learned 179We have tested many methods of extracting lexico-semantic relations (Chapters 3and 4). None of them ensures quality comparable to manual work. While the accuracywas often good, the problem was to find the limits. For example, Measures of SemanticRelatedness [MSR] produce continuous results. Defined for any pair of lemmas, theextraction based on lexico-syntactic patterns produces LU pairs that represents differentshades of some semantic relation. It is very hard to construct a general automaticmechanism that defines the border between those potential relation instances which arecorrelated with the linguist’s judgment, and those which are not. It becomes easierwhen we consider automatic expansion of an existing wordnet. The linguistic knowledgealready represented by the wordnet structures helps increase the trustworthinessof the automated additions. Consider the promising results of the WordNet Weaverapplication (Section 4.5.4).A core plWordNet should contain the upper levels of the hypernymy hierarchy, butit is very hard to construct it top-down without compromising the linguistic nature ofthe lexical network: one can unwittingly “slip” into an abstract ontology (taxonomy).More general LUs have few true hypernyms, and it is difficult to distinguish betweentheir direct and indirect hyponyms. Bottom-up work might be safer, but it too hasa drawback: the proper selection 7 of the more specific LUs in order to “activate” a widerange of more general LUs at the end of the process of core plWordNet construction.The problem is to make the selection in such a way that we can get an exhaustive setof the most general LUs if we just keep describing hypernyms of the specific LUs. Ourexperience suggests strong preference for the bottom-up approach. It worked especiallywell during the semi-automatic expansion phase.Our experiments with the WordNet Weaver [WNW], a tool for semi-automaticwordnet expansion (Sections 4.5.3-4.5.4), were generally encouraging. 8361 new lemmas,10537 new LUs, 8729 synsets and 11063 instances of lexico-semantic relationshave been added to the core plWordNet at the cost of 3.4 person-months. Every decisionassisted by WNW was verified by a coordinator, and many improvements weremade to the initial plWordNet structure.It is hard to separate the time spent on correcting the core plWordNet from the timespent on expanding it. WNW allowed us to discover many errors in the core plWordNetstructure: trying to attach new lemmas to the existing structure often brought out thedrawbacks of that structure. WNW’s suggestions are less helpful for gerunds (it is anopen problem how to reconcile gerund description by both MSRs and by pattern). Thepercentage of usefully suggested attachment varies across domains.We initially assumed a model of first generating part of the plWordNet structureand then correcting it semi-automatically. That did not work well at all. A linguistcould be lost if required to make hundreds of corrections in a continuously evolvingwordnet structure. It seems better to correct the proposed attachments one by one.7 Such selection is necessarily constrained, not the least by financial considerations.