A recursive statistical translation model

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ProceedingsoftheACLWorkshoponBuildingandUsingParallelTexts,pages199–207,AnnArbor,June2005.c󰀁AssociationforComputationalLinguistics,2005ARecursiveStatisticalTranslationModel∗

JuanMiguelVilar

Dpto.deLenguajesySistemas

Inform´aticos

UniversitatJaumeI

Castell´on(Spain)

jvilar@lsi.uji.esEnriqueVidal

Dpto.deSistemasInform´aticos

yComputaci´on

UniversidadPolit´ecnicadeValencia

InstitutoTecnol´ogicodeInform´atica

Valencia(Spain)

evidal@iti.upv.es

Abstract

Anewmodelforstatisticaltranslationis

presented.Anovelfeatureofthismodel

isthatthealignmentsitproducesarehier-

archicallyarranged.Thegenerativepro-

cessbeginsbysplittingtheinputsen-

tenceintwoparts.Eachofthepartsis

translatedbyarecursiveapplicationof

themodelandtheresultingtranslation

arethenconcatenated.Ifthesentence

issmallenough,asimplermodel(inour

caseIBM’smodel1)isapplied.

Thetrainingofthemodelisexplained.Fi-

nally,themodelisevaluatedusingthecor-

porafromalargevocabularysharedtask.

1Introduction

SupposeyouweretofindanEnglishtranslationfor

aSpanishsentence.Onepossibleapproachistoas-

sumethateveryEnglishsentenceisacandidatebut

thatdifferentEnglishsentenceshavedifferentprob-

abilitiesofbeingthecorrecttranslation.Then,the

translationtaskcanbedividedintwoparts:define

anadequateprobabilitydistributionthatanswersto

thequestion“giventhisEnglishsentence,whichis

theprobabilitythatitisagoodtranslationofthat

Spanishsentence?”;andusethatdistributioninor-

dertofindthemostlikelytranslationofyourinputsentence.

∗WorkpartiallysupportedbyBancaixathroughtheproject“SistemasInductivos,Estad´ısticosyEstructurales,paralaTra-ducci´onAutom´atica(Siesta)”.Thisapproachisreferredtoasthestatisticalap-

proachtomachinetranslation.Theusualapproach

istodefineanstatisticalmodelandtrainitsparame-

tersfromatrainingcorpusconsistinginpairsofsen-

tencesthatareknowntobetranslationofeachother.

Differentmodelshavebeenpresentedinthelitera-

ture,seeforinstance(Brownetal.,1993;Ochand

Ney,2004;Vidaletal.,1993;Vogeletal.,1996).

Mostofthemrelyontheconceptofalignment:a

mappingfromwordsorgroupsofwordsinasen-

tenceintowordsorgroupsintheother(inthecase

of(Vidaletal.,1993)themappinggoesfromrules

inagrammarforalanguageintorulesofagrammar

fortheotherlanguage).Thisconceptofalignment

hasbeenalsousedfortaskslikeauthomaticvocab-

ularyderivationandcorpusalignment(Daganetal.,

1993).

Anewstatisticalmodelisproposedinthispa-

per,whichwasinitiallyintroducedin(VilarTorres,

1998).Thismodelisdesignedsothatthealign-

mentbetweentwosentencescanbeseeninanstruc-

turedmanner:eachsentenceisdividedintwoparts

andtheyareputincorrespondence;theneachof

thosepartsissimilarlydividedandrelatedtoits

translation.Thisway,thealignmentcanbeseenas

atreestructurewhichalignsprogressivelysmaller

segmentsofthesentences.Thisrecursiveprocedure

givesitsnametothemodel:MAR,whichcomes

from“ModelodeAlineamientoRecursivo”,which

isSpanishfor“RecursiveAlignmentModel”.

Therestofthepaperisstructuredasfollows:af-

teracommentonpreviousworks,weintroducethe

notationthatwewillusethroughoutthepaper,then

webrieflyexplainthemodel1fromIBM,nextwe

199introduceourmodel,thenweexplaintheprocess

ofparameterestimation,andhowtousethemodel

totranslatenewtestsentences.Finally,wepresent

someexperimentsandresults,togetherwithconclu-

sions.

2Previousworks

Theinitialformulationoftheproposedmodel,

includingthetrainingprocedures,waspresented

in(VilarTorres,1998),alongwithpreliminaryex-

perimentsinasmalltranslationtaskwhichprovided

encouragingresults.

Thismodelsharessomesimilaritieswiththe

stochasticinversiontransductiongrammars(SITG)

presentedbyWuin(Wu,1997).Themainpoint

incommonisthetypeofpossiblealignmentscon-

sideredinbothmodels.Someoftheproperties

ofthesealignmentsarestudiedin(ZensandNey,

2003).However,theparametrizationsofSITGsand

theMARarecompletelydifferent.Thegenerative

processofSITGsproducessimultaneouslythein-

putandoutputsentencesandtheparametersofthe

modelrefertotherulesofthenonterminals.This

providesasymmetrytobothinputandoutputsen-

tences.Incontrast,ourmodelclearlydistinguishes

theinputandoutputsentencesandtheparameters

arebasedonobservablepropertiesofthestrings

(theirlengthsandthewordscomposingthem).On

theotherhand,theMARideaofsplittingthesen-

tencesuntilasimplestructureisfound,alsoap-

pearsintheDivisiveClusteringapproachpresented

in(Dengetal.,2004).Again,themaindifference

liesintheprobabilisticmodelingofthealignments.

InDivisiveClusteringauniformdistributiononthe

alignmentsisassumedwhileMARusesaexplicit

parametrization.

3Somenotation

Intherestofthepaper,weusethefollowingnota-

tion.Sentencesaretakenasconcatenationsofsym-

bols(words)andrepresentedusingaletteranda

smallbar,likein¯x.Theindividualwordsarede-

signedbythenameofthesentenceandasubindex

indicatingtheposition,so¯x=x1x2...xn.The

lengthofasentenceisindicatedby|¯x|.Segments