Semantic Role Labelling of Prepositional Phrases(2)

Abstract. In this paper, we propose a method for labelling prepositional phrases according to two different semantic role classifications, as contained in the Penn treebank and the CoNLL 2004 Semantic Role Labelling data set. Our results illustrate the dif

Fig.1.AnexampleoftheprepositionsemanticrolesinPennTeebank

2PrepositionSemanticRoleDisambiguationinPenn

Treebank

Signi cantnumbersofprepositionalphrases(PPs)inthePenntreebank[3]aretaggedwiththeirsemanticrolerelativetothegoverningverb.Forexample,Figure1,showsafragmentoftheparsetreeforthesentence:[Japan’sreservesofgold,convertibleforeigncurrencies,andspecialdrawingrights]fellbyahefty$1.82billioninOctoberto$84.29billion[theFinanceMinistrysaid],inwhichthethreePPsgovernedbytheverbfellaretaggedas,respectively:PP-EXT“extend”,meaninghowmuchofthereservefell;PP-TMP“temporal”,meaningwhenthereservefell,andPP-DIR“direction”,meaningthedirectionofthefall.

Accordingtoouranalysis,thereare143prepositionsemanticrolesinthetree-bank.However,manyofthesesemanticrolesareverysimilartooneanother;forexample,thefollowingsemanticroleswerefoundinthetreebank:PP-LOC,PP-LOC-1,PP-LOC-2,PP-LOC-3,PP-LOC-4,PP-LOC-5,PP-LOC-CLR,

PP-LOC-CLR-2,PP-LOC-CLR-TPC-1.Inspectionofthedatarevealednosystem-aticsemanticdi erencesbetweenthesePPtypes.Indeed,formostPPs,itwasimpossibletodistinguishthesubtypesofagivensuperclass(e.g.PP-LOCinourexample).WethereforedecidedtocollapsethePPsemanticrolesbasedontheir rstsemanticfeature.Forexample,allsemanticrolesthatstartwithPP-LOCarecollapsedtothesingleclassPP-LOC.Table1showsthedistributionofthecollapsedprepositionsemanticroles.

[4]describeasystemfordisambiguatingthesemanticrolesofprepositionsinthePenntreebankaccordingto7basicsemanticclasses.Intheirsystem,O’HaraandWeibeusedadecisiontreeclassi er,andthefollowingtypesoffeatures:–POStagsofsurroundingtokens:ThePOStagsofthetokensbeforeandafterthetargetprepositionwithinaprede nedwindowsize.InO’HaraandWiebe’swork,thiswindowsizeis2.–POStagofthetargetpreposition–Thetargetpreposition–Wordcollocation:Allthewordsinthesamesentenceasthetargetprepo-sition;eachwordistreatedasabinaryfeature.–Hypernymcollocation:TheWordNethypernyms[5]oftheopenclasswordsbeforeandafterthetargetprepositionwithinaprede nedwindowsize(setto5words);eachhypernymistreatedasabinaryfeature.