Phrasal Translation and Query Expansion Techniques for Cross-Language Information Retrieval

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PhrasalTranslationandQueryExpansionTechniquesforCross-LanguageInformation

Retrieval

LisaBallesterosandW.BruceCroft

balleste@cs.umass.edu,croft@cs.umass.edu

CenterforIntelligentInformationRetrieval

ComputerScienceDepartment

UniversityofMassachusetts

Amherst,MA01003-4610USA

Dictionarymethodsforcross-languageinformationretrieval

giveperformancebelowthatformono-lingualretrieval.

Failuretotranslatemulti-termphraseshasbeenshownto

beoneofthefactorsresponsiblefortheerrorsassociated

withdictionarymethods.First,westudytheimportanceof

phrasaltranslationforthisapproach.Second,weexplorethe

roleofphrasesinqueryexpansionvialocalcontextanalysis

andlocalfeedbackandshowhowtheycanbeusedtosignif-

icantlyreducetheerrorassociatedwithautomaticdictionary

translation.

ThedevelopmentofIRsystemsforlanguagesotherthan

Englishhasfocusedonbuildingmono-lingualsystems.In-

creasedavailabilityofon-linetextinlanguagesotherthan

Englishandincreasedmulti-nationalcollaborationhave

motivatedresearchincross-languageinformationretrieval

(CLIR)-thedevelopmentofsystemstoperformretrieval

acrosslanguages.

TherehavebeenthreemainapproachestoCLIR:transla-

tionviamachinetranslationtechniques[Rad94];parallelor

comparablecorpora-basedmethods[DD95a,LL90,SB96],

anddictionary-basedmethods[Sal72,Pev72,HG96,BC96].

Eachoftheseapproacheshasshownpromise,butalsohas

disadvantagesassociatedwithit.Resultssuggestthatim-

provementsgainedviamachinetranslationtechniquesmay

notoutweighthecostoflinguisticanalysis.Onedisadvan-

tageofmethodsbasedontheuseofparallelandaligned

corporaislackofresources:parallelcorporaarenotal-

waysreadilyavailableandthosethatareavailabletendto

berelativelysmallortocoveronlyasmallnumberofsub-

jects.Performanceisalsodependentonhowwellthecor-poraarealigned.Ourworktakesthethirdapproachand

appliesdictionary-basedmethods.

Automaticmachinereadabledictionary(MRD)query

translationleadstoadropineffectivenessof40-60%below

thatofmono-lingualretrieval[HG96,BC96].Thisisdue

primarilytothreefactors.First,specializedvocabularynot

containedinthedictionarywillnotbetranslated.Second,

dictionarytranslationsareinherentlyambiguousandaddex-

traneoustermstothequery.Third,failuretotranslatemulti-

termconceptsasphrasesreduceseffectiveness.

Wearedevelopingstrategiesforreducingtheerrorsas-

sociatedwithdictionary-basedmethodsandfocusonstrate-

gieswhichhavealowprocessingcostanddonotrequire

scarceresources.Thispaperexplorestheidentificationof

phrasesinqueriesandtheeffectivenessofsimplephrasal

translation.Inaddition,weinvestigatetheroleofphrasesin

queryexpansionbycomparingtwoapproaches,localfeed-

back[AF77]andLocalContextAnalysis[XC96],toex-

pandingqueriesatvariousstagesofthe“translation”pro-

cess.

Effectivesystemsformono-lingualinformationretrieval

havebeenavailableforseveralyears.Typically,research

intheareaofmulti-lingualinformationretrievalhasfocused

onincorporatingnewlanguagesintoexistingsystemstoal-

lowthemtoruninseveralmono-languageretrievalmodes.

Recently,greaterinterestinretrievalacrosslanguageshas

motivatedmoreworktostudythefactorsinvolvedinbuild-

ingaCLIRsystem.

Salton[Sal72]showedearlyonthatwithcarefullycon-

structedthesauri,cross-languageretrievalwasnearlyasef-

fectiveasmono-lingualretrieval.Thisstudywasgood,how-

everthetestcollectionwasverysmallbycurrentstandards

anditisunrealistictomanuallyindexlargerdatabases.

LandauerandLittman[LL90]havealsoproposeda

methodforcross-languageretrieval.LatentSemanticIndex-

ing(LSI)[FDD88]wasusedtocreateamultidimensional

indexingspaceforaparallelcorpusofEnglishdocuments

andtheirFrenchtranslations.Theirmethodhasbeensuc-cessfulatthetaskofretrievingaquery’stranslation,inre-

sponsetothatquery.Howeverthecollectionusedwassmall,

containing2482paragraph-lengthdocumentsfromCana-

dianParliamentaryproceedingsandnoresultsofitseffec-

tivenessonthetraditionalretrievaltaskhavebeenreported.

Themethodalsoreliesontheuseofparallelcorporawhich

arenotalwaysreadilyavailable.

Anothermethodthatreliesonparallelandalignedcor-

porahasbeensuggestedbyDunningandDavis[DD93].

Theirmethodisbasedonthevectorspacemodelandin-

volvesthelineartransformationoftherepresentationofa

queryinonelanguagetoitscorrespondingrepresentationin

anotherlanguage.Thetransformationisdonebyreduction

ofthedocumentspacetogenerateatranslationmatrix.They

havehadsomesuccessinefficientlyestimatingthetrans-

lationmatrixandresultsofteststoestimateitsqualityare

promising.Furthertestsoftheeffectivenessofthemethod

havebeenlimitedbyitscomputationalcomplexity.

DavisandDunning[DD95a,DD95b]havealsodevel-

opedseveralotherapproachestoquerytranslation,which

theytestedontheTRECISMSpanishqueriesandcollec-

tion.TwooftheserelyontheuseofaSpanish-English

parallelcorpusandoneusesevolutionaryprogrammingfor

queryoptimization.Inthefirstoftheparallelcorpusap-