From: AAAI Technical Report WS-92-01. Compilation copyright © 1992, AAAI (www.aaai.org). All rights reserved. Example-Based NLP Techniques - A Case Study of Machine Translation Eiichiro SUMITA and Hitoshi HDA ATRInterpreting Telephony Research Laboratories Hikaridai, Seika, Souraku, Kyoto 619-02, JAPAN e-mail:{ sumita, iida}@atr-la.atr.co.jp Abstract This paper proposes Example-Based NLP (EBNLP)which uses EXAMPLES (pairs of types of data, e.g. a sentence and its parse) extracted from a corpus and the DISTANCE between examples, explaining Example-Based MachineTranslation (EBMT) in particular. EBMT prototype has been implementedto deal with frequent and polysemous linguistic phenomena such as Japaneseverbs, case particles and English prepositions. The average success rate of each phenomenonand interesting relationships betweensuccess rate and example databaseparametersare presented. knowledgebases, and the remarkablesuccess of speech recognitionusingspeechdatabaseshas createda needfor very large linguistic corpora, drawingattention to corpus-basedNIP. Corpus-hasedNLPprojects vary according to the followingparameters: ¯ Is the corpusmonolingnal or bilingual? ¯ Howstructured is the corpus?Is it in a string, word sequence,syntacticstructure, or semanticstructure? ¯ Whatis the mechanism using the corpus? ¯ Is knowledge extractedfromthe corpusor is the corpus useddirectly? ¯ Whatis quality andsize of the corpus? Recently, the increase of electronically stored documents,the progress of hardwaresuch as memory and parallel computers’,the development of very large EBNLPcan be explained while observing corpus-based NLP. A monolingual corpus is related to analysis and synthesis research. Basedon the co-occurrenceor n-gramsextracted froma monolingualwordor string level corpus, a wide range of studies such as the semanticclassification of words [Hirschman et al., 1975;Hindle,1990], spell checking, andthe correction of speechrecognitionerrors havebeen made. Moreover, Smadja has begun new research collecting collocations using n-grams and a parser [Smadja,1991]. Frequencyof dependencybetweenwords extracted from a monolingualsyntactic structure level corpus, is used to eliminate syntactic ambiguity [’rsutsumi and Tsutsumi,1989; Nagao,K., 1990]. A bilingual corpus is related to translation research? A statistical method for aligningsentencesin a bilingual string corpus,has been proposed by Gale and Church[Galeand Church, 1991]. Astatistical methodfor translating sentencesbasedon mutualinformationin a bilingual wordcorpus, has been proposed by Brown et al.[Brown et al., 1988]. Independentof this, EBMT, whichutilizes bilingual examples (pairs of a sourcesentenceandits translation) and the distance betweenexamples,has emerged[Nagan, M., 1984; Sadler, 1989; Sato, 1991; Sumitaand Iida, 1991b;Furuseand Iida, 1992;Watanabe,1991]. It has been proven that massively parallel computers [Kitano, 1991] and parallel computers [SumitaandIida, 1991a]are effective in acceleratingthe retrieval froma very large corpus. " The bilingual corpus is useful for not only translation but also analysis. A methodfor eliminating syntactic ambiguityusing bilingual exampleshas been proposed[Furuse andIida, 1992]. 1 Introduction Recently,the constructionanduse of very large corpora has begunto increase [Walker,1990]andhas introduced a variety of corpus-basedNLPresearch projects. The authors propose Example-Based NLP (EBNLP) which uses EXAMPLES (pairs of two types of data, e.g. a sentenceandits parse)extractedfroma corpusand the DISTANCE betweenexamples. This paper explains Example-Based Machine Translation (EBMT) in detail. Basedon the experimentspresented in the paper, the authors demonstrate that Example-Based Machine Translationis effective in translating several linguistic phenomenaand also present interesting relationships betweensuccessrate andexampledatabase parameters. 2 Background The authors’ opinion is as follows: (1) The approachby Brownet aL whichacquires all necessary information from a bilingual word level corpus is expectedto reducethe effectiveness for translation of languagepairs, e.g. English and Japanese, whichare drastically different from each other in wordsand grammars, for example, word order, (2) Sato (his MBT2 model)and Sadler supposethat translation units whichare equivalent to each other from the point of translationare linked.Thereare still manydifficulties to overcomein order to build linked bilingual example databases. Unlikethese approaches,the authors assume hybrid architectures which incorporate EBMT as a subroutine into conventional machine translation systems.’ Weprepareda databaseof formattedexamples for several linguistic phenomena for this paper. Weaim to clarify what kind of phenomena EBMT is suited to andillustrate the relationshipsbetweensuccessrate and the exampledatabaseparameters. Anotherinstance of EBMT is the retrieval of similar sentencesfor translation aid. It enablesa person to retrieve sentences from an accumulatedtranslation exampleswhichare similar to the sentencehe wantsto translate. ETOCretrieves similar sentences by generalizing input incrementally[Sumitaand Tsutsumi, 1988]. CTMmeasures the distance by counting the numberof characters whichco-occurin both the input and the example[Sato,1992]. Eventhoughthe corpus is monolingual,based on pairs of twotypesof data, e.g. a string andits parse as examplesand the distance betweenthe examples,a variety of research will emerge.Theauthors generally call them EBNLP. EBNLPmakes much of individualities in linguistic phenomena.At the start, it does not acquire abstract knowledge from corpus but utilizes the corpus directly. Research on compressingthe exampledatabase while maintaining system performanceis an important future task for memory size and processingspeed. This research formsonepart of ATR’sresearch whichaims to realize interpreting telephony. TheATR corpus contains conversationsabout registering for an international conference. ATRhas collected about 17,000 Japanese sentences (about 270,000words) analyze linguistic phenomena and extract statistic information.EachJapanesesentenceis morphologically and syntactically analyzedand its English translation ) Theauthors assumethat linguistic phenomena can be divided into two parts: a regular part whichis describedwell byrules andan irregular part whichis not describedwell by rules. Theformerrelates to modelorientedNLPandthe latter relates to data-orientedNLP. It is morerealistic to incorporate a data-oriented approachwith a model-orientedapproachthan to use a single approachexclusively. 82 Atmehed[Ehara et al., 1990]. 3 Outline of EBMT Recentmachinetranslation technologyhas reachedthe level whereseveral systemshave been commercialized and are used daily. However, the conventional technologyis not satisfactory in somepoints such as target wordselection. This paper addresses the word selection problemsof several phenomenaand proves EBMT feasibifity throughsuccessful experiments. Example-BasedMachine Translation (EBMT) was proposed by Nagao[Nagao,M., 1984] in order to overcomeproblemsinherent in conventional Machine Translation. In EBMT, (1) a tin!abase whichconsists examplesis prepared for translation knowledge;(2) examplewhosesourcepart is similar to the input phrase or sentenceis retrieved fromthe exampledatabase;(3) the translation is obtainedby replacingof corresponding wordsin the target expressionof the retrievedexample. (1) Analysis (4) Example Database (2) Example-Based Transfer (5) Thesaurus (3) Generation Figure 3.1 EBMTConfiguration As shown in Figure 3.1, EBMTuses two databases, i.e., an exampledatabase and a thesaurus’. Examples (pairs of a source phraseand its translation) are abstracted from the bilingual corpus. For convenience, a translation pattern (tp), e.g., for English adverbial preposition "in", "N2~=- VI’, "N2"~ VI", "N2¢ VI" is abstracted from an exampleand stored with the examplein the exampledatabase. Table 4.1 ’ The hierarchy of the Japanese thesaurus is in accordance with the thesaurus of everyday Japanese [Ohnoand Hamanishi, 1984], the hierarchy of the English thesaurus is in accordance with the LONGMAN LEXICON[McArthur, 1981]. To what extent EBMT is sensitive to the hierarchy of the thesaurus is an open andinterestingissue. showsthe translation pattern distributionof "in". The distance measured by the summation of the word distances multiplied by the weight of each word. It is assumedthat the input andthe examplein the exampledatabase(referred to as andE) are representedin the list of words’syntacticand semanticattribute values(referred to as k and Ek) for eachphrase. Thedistanceis calculatedac-cording-tbthe expression(1). Different measuresof worddistance and wordweighthave been proposedas shownin footnotes 5 and6. (1) d(I,E)ffiY~ d(Ik,Ek) k k Here, weexplain our measure[Sumita and Iida, 1991b]briefly. Worddistanced(Ik,Ek.) is proportionalto the location of the conceptwli~ch zs called the Most sSpecific Common Abstraction. I ".tl i A,B C of tpwhen Ek=Ik) EBMT’s advantagesare as follows: *Byadding examples,the user himself canimprovethe quality of EBMT. *Basedon a bilingual corpuswhichrealizes translators’ expertise, EBMT can generatea high-qualitytranslation. *Thedistance functions as an indicator of translation reliability.’ *Even though the example database is very large, acceleration is possible using indexing and parallel computing. Prepositions 4.1 English Prepositions *English prepositions occur frequently. Prepositionsare the basicdevicesused in constructing English verb and nounphrases and occur frequently. Prepositionsdealt with in this paper, i.e., "of", "to", "for", "in", "on", "at", "from","by", "with"are within the almost top fifty.’ The corpus has about 270,000 words. Thetotal numberof examplesis about 3,300. -English prepositions are highly polysemous. LONGMANDICTIONARY OF CONTEMPORARY ENGLISH[Longman, 1978] lists 21 senses for the preposition"in". AnnetteHerskovitslists 11 senses for the preposition "in" of locative expression[Herskovits, 1986]. *English prepositions together with the following nounsmodifypreceding verbs or nouns. In this paper wecall the former adverbial usage (1) and the latter adnominalusage (2). I I .I [I /I II \ I D 2(2)Wk=,,/Y. (freq. tp 4 EBMTfor Thesaurus Root~J= d(A,C) summationof the square of frequency of translation pattern(tp) in the subset of the exampledatabasewhere Ek=I k. E Figure 3.2 ThesaurusHierarchy and Distance Thedistance varies from 0 to 1. In Figure 3.2, A, B, C, D and E represent words, and squares represent concepts,to whichcorresponding distancesare attached. Wordweight is computedaccording to the followingexpression.* Wordweightis the root of the s Sato[Sato, 1991]andSadler[Sadler,1989]compute worddistanceusing feature vectors whichconsist of cooccurrencebequenciesin the example~!,base. ’ Sato computes word weight using an information-basedestimation method[Sato,1991]. The weight are invariable for any input. Consideringthe translation of "A B C" which consists of three components,"A", "B", and "C", this methodcan only determine a general tendency for target expression selection, e.g., "In general, Ais the mostimportantof the three components,"and thus A should receive the highest weight. However,in individual cases, "B or C can often be the mostimportantof the three." 83 (1) I’ll seeyou{inOsaka}. V P N (2) I’m workingfor a comvanvi~ Kvotol. N P N ’ Asreportedin [SumitaandIida,1991b],generally, the smaller the distance, the better the quality. Moreover,the distance functions as an indicator of translation reliability. In the reportedexperiment,(1) the cases whered<=0.5,the success rate is 82%.(2) the cases whered>0.5, the success rate is 37%.The difference betweenthese two cases is remarkable. ’ Theseprepositionsoccurfrequentlyin other domains,e.g., computermanuals,as well. The rates of the two usages vary from preposition to preposition (Figure 4.1). For example, the rates adnominalusage of "of" and adverbial usage of "to", "by", and "with"arc great(over90%).Otherprepositions are less biased. Both adverbial usage and adnominal usageare important,so the two are dealt with uniformly by our approach. Prepositions of in to ¯ adverbial usage r’l adnominal usage by Distribution (V1 In N2) tp N2 ~- V1 N2 "~ V1 N2 ¢ VI" total [] with i | | I ! l I i I I 100200 300400500 600700800 9001000 Number Figure 4.1 Adverbial Usagevs. Adnominal Usage Prepositions at ¯ Thefollowing samplecorrespondences showhowthe adverbialusage of "in" corresponds to the Japanese case particles,"~, ~ andso on.’ ......... live in Kyoto plan - in March rate 0.630 0.359 0.010 1.000 0 [] ni [] de IRI ni-kanshite [] e(zero) == ..................... I , , , , , , , , , I 0 10 20 30 40 50 60 70 80 90 100 hold - in Kyoto meet ~ in Kyoto frequency 242 138 4 384 Japanesecase particlesarebothfrequent andpolysemous as well. The correspondences between English prepositions and Japanese case particles are not one-to-one but complicated (Figure4.2). contrast, the correspondences between English prepositionsand Frenchprepositions are almostone-toone, e.g., "at" and"a","in" and"clans", "on" and"sur" ’= correspond eachother. on for at from Table 4.1 Translation Pattern Adverbial Particles --- % TM [Kyo~---’], [hold] g~$~-3 [Kyot-6"], [meet] Figure 4.2 Correspondence between English Prepositions and Japanese Particles (Adverbial Usage) [Kyo~--’], [live] --- 3~J ~:-:~:~ [Ma~’-h], [plan] meet- in the afternoon [afte~-oon],[meet] leave - in 10 minutes -- lO~-e$~3’- ~ [10 m’~utes],[leave] Table 4.1 shows the distribution correspondence, i.e., translation pauem(tp). 4.2 Experiment of the 4.2.1 Adverbial Usage "in" Examplesare represented as a simple fixed format consisting of three elements of the form (VERB PREP I| ’ Typicalcase particles are as follows:~¢(ga), (o), t~-(ni), "P(de),~ (to), "~(e), t~-[~L.’C(ni-kanshite), and so on. Romanization is parenthesizedhere. ,o Englishequivalences are bracketedthroughthis paper. 84 ¢ meansno case particle. ~2 In fact, there are manyexceptionsto this simple correspondence.In order to explain the correspondence, Nathalie Japkowiczand Janyce M. Wiebehave proposed a translation method based on conceptualization [Japkowicz and Wiebe, 1991]. Cornelia ZelinskyWibbelthas proposeda similar approachfor English and German[Zelinsky-Wibbelt’1990]. NOUN),such as (see in Osaka). Conjecture that Iranslation of adverbial prepositions dependsonly on the following nouns is not true, because neglecting the verb reduces the success rate. Wehave to refer to both verb and noun, word weights change on a case-by-case basis. The success rate was 89.8%. 4.2.2 General Tendency Figure 4.3 summarizesadverbial usage, Figure 4A summarizes adnominal usage." Successrate make changes at once -- ~,~, ~.~T 7o [immediately] [makechanges] ¯ Informationwithin the fixed format is not sufficient to determine the sense. For example, "be in Osaka" is wanslated differently ":J~:J~ ~:- & 70" and "~l~ "C &7o" depending on the subject of "be".This can be eliminated by using larger exampleswhennecessary." The hotel is in Osaka--* ~ ~’)//W 5~[~ ~=- ~ 70 [hotel] [Osaka] [be] The party is in Osaka--* /¢-- Y" 4 ~~ :)~:~’~ [070 lpartyI [Osaka] [be] 100 l 80 4.2.3 Relationship I, database 60 40 between success and This section outlines the relationships between the success rate and exampledatabase parameters. In Figures 4.5, 4.6, and 4.7, each square represents each preposition, e.g., adnominal"of", adverbial "in". 2O 0 of in on to for at from by with Prepositions ¯ In general, the more examples we have, the better the quality as reported in a previous paper[Sumita and Iida, 1991b]. However, the relationship between success rate and the number of examples is peculiar to each phenomenon.There is no commonequation applicable for all phenomena(Figure 4.5). Figure 4.3 Adverbial SuccessRate Successrate 100 Success rate y = 91.679- 1.2239a-3x R^2- 0.002 8O 60 ’°°l: ¢ mra m 40 [] 90 ! 20 80 0 of in on to for 0 at from by with Prepositions rl B ~1 , . 200 Figure 4.5 Figure 4.4 Adnominal Success Rate Major causes of failure ¯ NOsimilar examples. Thesefailures can be eliminated by addingexamples. ¯ Idiomatic expressions. Thesefailures can be eliminated by exception handling or by adding examples. [] , ,. , . , . , 400 600 800 1000 Number of Examples Success Rate per Number of Examples " If an example element does not have sufficient influence on translation, its weight is very low. For example, weight of "be" is lower than others. ,s If the number of examples is small (in our experiment of prepositions, under 20), the success rate is not stable, e.g., adverbial "of" and adnominal"to" and "with" are not very successful, while adnominal"by" is translated completely. Adverbial "of" and adnominal "to", "by", and "with" are neglected in this section. " Adverbial "of" and adnominal "to", "by", and "with" are shaded in Figures 4.3 and 4.4 as they have too few examples for their success rates to be meaningful. 85 ¯ In general, the moretranslation patterns, the worsethe quality. Figure 4.6 showsthe relationship the number of translation patterns andsuccessrate. Successrate Content Word y - 98.352- 4.4187xR^2- 0.220 m 100 90"t 80 0 m 2 content wordssuch as verbs andnounsand so on and (2) functionwordssuchas prepositionsandparticles. B [] / ~ 4 ~ns 6 For verbs particularly, there is one-to-manymapping" from the source languageword to the target language wordand to select the appropriate target languageword is an importanttask. Number of Translation 8 -~ )PP~: [milk] Y, -- 7" ~ [soup] ~" [medicine] ~ ~: [cigarette] 10 Figure 4.6 Success Rate per Number of Translation Patterns ¯ In general, the higher the weight, the better the quality. Figure 4.7 shows the relationship the weight and success rate. Theword weight defined by expression(2) in section 3 is computedin subset of the exampledatabase. In contrast, this weight is computedas the root of the summation of the square of frequency of translation pattern(tp) in the whole exampledatabase. Successrate y - 69.341+ 29.725xR^2- 0.602 100 eig ht 80 ~-’l~;~mi-~~ 0.4 0.5 0.6 0.7 ¢30 -"* drink milk [drink/eat/take/smoke] 0 0 --* eat soup [drink/eat/take/smoke] 0 0 --* take medicine [drink/eat/take/smoke] 0 ~r -" smoke [drink/eat/take/smoke] Because EBMT does not use rules based on semantic markers but uses examples directly and measuresthe distance based on a thesaurus, EBMT is expectedto performthe finer wordselection. Function Word Anadverbial preposition is a typical function word.It together with the followingnounsmodifythe preceding verbs. Tables 5.1 and 5.2 classify function words according to whetherthe modifier" is a nounor verb and whether the modificand is a noun or verb. As exemplifiedin section 4, in general, function wordsare frequent and polysemous (in manycases, they haveoneto-manymappings). Consequently,they are important in translation. Table 5.1 Classification of Function Words (English) 0.8 0.9 1.0 N V Figure 4,7 Success Rate per Weight 5 EBMTfor Other Phenomena N adnominal preps e.g. theobjectof t h u conference adverbial preps e.g.meet at t h¯ station V relativeclauses etc. e.g.the manwho walks conjunctions e.g.walkwhile listening This section reports translation of other linguistic phenomena whichwehaveconductedto date. 5.1 Other Phenomena Here, an outline of linguistic phenomenais given. EBMT is applicable to various difficult translation problems,as reported in the paper[Sumitaand Iida, 19 91b]. Here, wewill explain phenomena,dividing words into two classes froma grammaticalpoint of view: (1) 86 ,6 In the ATRcorpus, one-to-many verb are 36. l%(type)and 81.5%(token). "Modifiersare boldfacedin the tables. Table 5.2 Classification of Function Words (Japanese)" N adnominal case particles N V adverbial caseparticles e.g.~’~ ") [conference],[object][station],[meet] V relativeclauses etc. e.g.~ < _zJ~ [walk],[man] conjunctions e.g.~~ ~’,~:0~ G~ < [listen],[walk] Acknowledgements Theauthors gratefully acknowledge the help providedby Makoto NAGAOand Satoshi SATO at Kyoto University, and by Akira KUREMATSU, Osamu FURUSE,and other members at ATRInterpreting TelephonyResearchLaboratories. $.2 Experiment Wehavetested verbsas contentwordandthe top of Tables5.1 and5.2 as functionwords.Withthe average successrates shownbelow, EBMT is shownto be effectivefor bothcontentwordsandfunctionwords. (1) (2) (3) (4) (5) verb" adnominalprepositions adverbialprepositions ~ adnominalcase panicles ~’ adverbialcaseparticles (JE) (F_J) (EJ) (JE) (JE) 87% 87% 90% 78% 79% 6 Conclusion This paper has proposed the Example-Based NLP (EBNLP)which uses EXAMPLES (pairs of two types of data, e.g. a sentenceandits parse) extracted from corpus and DISTANCE between the examples. To show EBNLPfeasibility, an Example-Based Machine Translation (EBMT)prototype using fixed format exampleswhich deals with frequent and polysemous linguistic phenomenasuch as Japanese verbs, case particles and English prepositions wasimplemented. The average success rates were high, i.e., about "Weassumethat there is zero wordfor cases such as <ltA. 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