Unsupervised Approaches to Sequence Tagging, Morphology ...rezab/papers/unsupnlp_slides.pdf ·...

UnsupervisedApproachestoSequenceTagging,MorphologyInduction,andLexicalResource

AcquisitionRezaBosaghzadeh&NathanSchneider

LS2~1December2008

UnsupervisedMethods– SequenceLabeling(Part‐of‐SpeechTagging)

– MorphologyInduction

– LexicalResourceAcquisition

.

She ran to the station quickly

pronoun verb preposition det noun adverb

un‐supervise‐dlearn‐ing

ContrastiveEstimationSmith&Eisner(2005)

•  Alreadydiscussedinclass•  Keyidea:exploitsimplicitnegativeevidence

– Mutatingtrainingexamplesoftengivesungrammatical(negative)sentences

– Duringtraining,shiftprobabilitymassfromgeneratednegativeexamplestogivenpositiveexamples

•  BUT:Requiresataggingdictionary,i.e.alistofpossibletagsforeachwordtype

Prototype‐driventaggingHaghighi&Klein(2006)

+

PrototypesTargetLabel

UnlabeledData

PrototypeList

AnnotatedData

slidecourtesyHaghighi&Klein


Newlyremodeled2Bdrms/1Bath,spaciousupperunit,locatedinHilltopMallarea.Walkingdistancetoshopping,publictransportation,schoolsandpark.Paidwaterandgarbage.Nodogsallowed.


PrototypeList

NN VBN CC JJ CD PUNC

IN NNS IN NNP RB DET

NN president IN of

VBD said NNS shares

CC and TO to

NNP Mr. PUNC .

JJ new CD million

DET the VBP are

EnglishPOS


Prototypes


FEATURE kitchen, laundry

LOCATION near, close TERMS paid, utilities SIZE large, feet RESTRICT cat, smoking

InformationExtraction:ClassifiedAds

FeaturesLocationTermsRestrictSize

PrototypeList



•  Trigramtagger,samefeaturesas(Smith&Eisner2005)– Wordtype,suffixesuptolength3,contains‐hyphen,contains‐digit,initialcapitalization

•  Tieeachwordtoitsmostsimilarprototype,usingcontext‐basedsimilaritytechnique(Schütze1993)–  SVDdimensionalityreduction–  Cosinesimilaritybetweencontextvectors

slideadaptedfromHaghighi&Klein


Pros•  Doesn’trequiretaggingdictionaryCons•  Stillneedatagset•  Maybehardtochoosegoodprototypes

UnsupervisedPOStaggingTheStateoftheArt

Bestsupervisedresult(CRF):99.5%!




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UnsupervisedApproachestoMorphology

•  Morphologyreferstotheinternalstructureofwords– Amorphemeisaminimalmeaningfullinguisticunit

– Morphemesegmentationistheprocessofdividingwordsintotheircomponentmorphemes

un‐supervise‐dlearn‐ing– Wordsegmentationistheprocessoffindingwordboundariesinastreamofspeechortextunsupervised_learning_of_natural_language

ParaMor:MorphologicalparadigmsMonsonetal.(2007,2008)

•  Learnsinflectionalparadigmsfromrawtext– Requiresonlyalistofwordtypesfromacorpus– Looksatwordcountsofsubstrings,andproposes(stem,suffix)pairingsbasedontypefrequency

•  3‐stagealgorithm– Stage1:Candidateparadigmsbasedonfrequencies

– Stages2‐3:Refinementofparadigmsetviamergingandfiltering

•  Paradigmscanbeusedformorphemesegmentationorstemming


speak dance buyhablar bailar comprarhablo bailo comprohablamos bailamos compramoshablan bailan compran… … …

•  AsamplingofSpanishverbconjugations(inflections)



•  Aproposedparadigm(correct):stems{habl,bail,compr}andsuffixes{‐ar,‐o,‐amos,‐an}


•  Twosubsequentstages:– Filteringoutspuriousparadigms(e.g.withincorrectsegmentations)

– Mergingpartialparadigmstoovercomesparsity:smoothing


speak dancehablar bailarhablo bailohablamos bailamoshablan bailan… …

•  Forcertainsub‐setsofverbs,thealgorithmmayproposeparadigmswithspuriousseg‐mentations,liketheoneatleft

•  Thefilteringstageofthealgorithmweedsouttheseincorrectparadigms


•  Whatifnotallconjugationswereinthecorpus?

speak dance buyhablar bailar comprar

bailo comprohablamos bailamos compramoshablan… … …


•  Anotherstageofthealgorithmmergestheseoverlappingpartialparadigmsviaclustering

speak dance buyhablar bailar comprar

bailo comprohablamos bailamos compramoshablan… … …



•  Thisamountstosmoothing,or“hallucinating”out‐of‐vocabularyitems


•  Heuristic‐based,deterministicalgorithmcanlearninflectionalparadigmsfromrawtext

•  Currently,ParaMorassumessuffix‐basedmorphology

•  Paradigmscanbeusedstraightforwardlytopredictsegmentations– CombiningtheoutputsofParaMorandMorfessor(anothersystem)wonthesegmentationtaskatMorphoChallenge2008foreverylanguage:English,Arabic,Turkish,German,andFinnish

•  Wordsegmentationresults–comparison

•  SeeNarges&Andreas’spresentationformoreonthismodel

Goldwateretal.UnigramDP

Goldwateretal.BigramHDP

BayesianwordsegmentationGoldwateretal.(2006;insubmission)

tablefromGoldwateretal.(insubmission)

MultilingualmorphemesegmentationSnyder&Barzilay(2008)

speakrs speaktuhablar parlerhablo parlehablamos parlonshablan parlent… …

•  Abstractmorphemescrosslanguages:(ar,er),(o,e),(amos,ons),(an,ent),(habl,parl)

•  Considersparallelphrasesandtriestofindmorphemecorrespondences

•  Straymorphemesdon’tcorrespondacrosslanguages

MorphologyPapers:Inputs&Outputs

•  Whatdoes“unsupervised”meanforeachapproach?




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BilinguallexiconsfrommonolingualcorporaHaghighietal.(2008)

SourceText

TargetText

Matching

m state

world

name

SourceWords

s

nation

estado

política

TargetWords

t

mundo

nombre

diagramcourtesyHaghighietal.UsedavariantofCCA(CanonicalCorrelationAnalysis)

state

Orthographic Features 1.0

1.0

1.0

#st

tat te#

5.0

20.0

10.0

Context Features

world politics society

SourceText

estado

Orthographic Features 1.0

1.0

1.0

#es

sta do#

10.0

17.0

6.0

Context Features

mundo politica sociedad

TargetText

slidecourtesyHaghighietal.

BilingualLexiconsfromMonolingualCorporaHaghighietal.(2008)

DataRepresentation

FeatureExperiments

61.1

80.1 80.289.0

0

25

50

75

100

EditDist Ortho Context MCCA

Precision

•  MCCA:Orthographicandcontextfeatures

4kEN‐ESWikipediaArticlesslidecourtesyHaghighietal.

NarrativeeventsChambers&Jurafsky(2008)

•  Givenacorpus,identifiesrelatedeventsthatconstitutea“narrative”and(whenpossible)predicttheirtypicaltemporalordering– E.g.:NOPQPRSTUOVWXNYZPVRnarrative,withverbs:arrest,accuse,plead,testify,acquit/convict

•  Keyinsight:relatedeventstendtoshareaparticipantinadocument– Thecommonparticipantmayfilldifferentsyntactic/semanticroleswithrespecttoverbs:arrest.V\]XNZ,accuse.V\]XNZ,plead.WY\]XNZ

NarrativeeventsChambers&Jurafsky(2008)

•  Atemporalclassifiercanreconstructpairwisecanonicaleventorderings,producingadirectedgraphforeachnarrative

StatisticalverblexiconGrenager&Manning(2006)

•  Fromdependencyparses,agenerativemodelpredictsforeachverb:– PropBank‐stylesemanticroles:wux0,wux1,etc.(donotnecessarilycorrespondacrossverbs)

– Theroles’syntacticrealizations,e.g.:

•  Usedforsemanticrolelabeling

He gave me a cookie

subj ARG0

verb give

np#1 ARG2

np#2 ARG1

He gave a cookie to me

subj ARG0

verb give

np#2 ARG1

pp_to ARG2

“Semanticity”:Ourproposedscaleofsemanticrichness

•  text<POS<syntax/morphology/alignments<coreference/semanticroles/temporalordering<translations/narrativeeventsequences

•  Wescoreeachmodel’sinputsandoutputsonthisscale,andcalltheinput‐to‐outputincrease“semanticgain”– Haghighietal.’sbilinguallexiconinductionwinsinthisrespect,goingfromrawtexttolexicaltranslations

SemanticGain:ComparisonofMethods

Robustnesstolanguagevariation•  AbouthalfofthepapersweexaminedhadEnglish‐onlyevaluations

•  Weconsideredwhichtechniquesweremostadaptabletoother(esp.resource‐poor)languages.Twomainfactors:– Relianceonexistingtools/resourcesforpreprocessing(parsers,coreferenceresolvers,…)

– Anylinguisticspecificityinthemodel(e.g.suffix‐basedmorphology)

SummaryWeexaminedthreeareasofunsupervisedNLP:

1.   Sequencetagging:HowcanwepredictPOS(ortopic)tagsforwordsinsequence?

2.   Morphology:Howarewordsputtogetherfrommorphemes(andhowcanwebreakthemapart)?

3.   Lexicalresources:Howcanweidentifylexicaltranslations,semanticrolesandargumentframes,ornarrativeeventsequencesfromtext?

Ineightrecentpaperswefoundavarietyofapproaches,includingheuristicalgorithms,Bayesianmethods,andEM‐styletechniques.

ThankstoNoahandKevinfortheirfeedbackonthepaper;AndreasandNargesfortheir

collaborationonthepresentations;andallofyouforgivingusyourattention!

Questions?


hablar bailar

hablo bailo

hablamos bailamos

hablan bailan

subj=give.wux0verb=givenp#1=give.wux2np#2=give.wux1

PrototypesTargetLabel

ImprovementIdeas

•  POSTagging:Learnthetagset•  Morphology:Non‐agglomerativeMorphology,Alsoparses

•  LexicalResources:Trywordclasses

•  All:Languagevariability

Unsupervised Approaches to Sequence Tagging, Morphology ...rezab/papers/unsupnlp_slides.pdf ·...

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