Semantic Role Labelling of Prepositional Phrases

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

SemanticRoleLabellingofPrepositionalPhrases

PatrickYeandTimothyBaldwin

DepartmentofComputerScienceandSoftwareEngineering

UniversityofMelbourne,VIC3010,Australia

{jingy,tim}@cs.mu.oz.au

Abstract.Inthispaper,weproposeamethodforlabellingpreposi-tionalphrasesaccordingtotwodi erentsemanticroleclassi cations,ascontainedinthePenntreebankandtheCoNLL2004SemanticRoleLabellingdataset.Ourresultsillustratethedi cultiesindeterminingprepositionsemantics,butalsodemonstratethepotentialforPPseman-ticrolelabellingtoimprovetheperformanceofaholisticsemanticrolelabellingsystem.

1Introduction

Prepositionalphrases(PPs)arebothcommonandsemanticallyvariedinopenEnglishtext.WhilePPscanoccurasbothcomplementsandadjunctstoverbs

[1]andalsoascomplementstonouns[2],thesemanticsofagivenPPcanof-tenbepredictedwithreasonablereliabilityindependentofcontext.Consider,forexample,thePPtothecar:ourexpectationwouldbeforittooccurasadirectionaladjunct,andonlyininstancessuchasrefertothecarwouldweseeasigni cantdivergencefromthissemantics,althoughhere,theimmediatecontextofthePPwouldgiveusanimmediatesenseofthesemanticshift.Basedonthisobservation,wemayconsiderthepossibilityofconstructingasemantictaggerspeci callyforPPs,whichusestheimmediatecontextofthePPtoarriveatasemanticanalysis.ItisthistaskoftargetedPPsemanticrolelabellingthatwetargetinthispaper.

APPsemanticrolelabellerwouldallowustotakeadocumentandidentifyalladjunctPPswiththeirsemantics.Wewouldexpectthistoincludealargeportionoflocativeandtemporalexpressions,e.g.,inthedocument,providingvaluabledatafortaskssuchasinformationextractionandquestionanswering.IndeedourinitialforayintoPPsemanticrolelabellingrelatestoaninterestingeoparsing,andtherealisationoftheimportanceofPPsinidentifyingandclassifyingspatialreferences.

ThecontributionsofthispaperaretoproposeamethodforPPsemanticrolelabelling,andevaluateitsperformanceoverboththePenntreebank(includingcomparativeevaluationwithpreviouswork)andalsothedatafromtheCoNLLSemanticRoleLabellingsharedtask.Aspartofthisprocess,weidentifythelevelofcomplementarityofadedicatedPPsemanticrolelabellerwithaconven-tionalholisticsemanticrolelabeller,suggestingPPsemanticrolelabellingasapotentialavenueforboostingtheperformanceofexistingsystems.