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CS460/626 : Natural Language Processing/Speech, NLP and the Web (Lecture 10, 11–MT approaches) Pushpak Bhattacharyya CSE Dept., IIT Bombay 25 th Jan and.

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Presentation on theme: "CS460/626 : Natural Language Processing/Speech, NLP and the Web (Lecture 10, 11–MT approaches) Pushpak Bhattacharyya CSE Dept., IIT Bombay 25 th Jan and."— Presentation transcript:

1 CS460/626 : Natural Language Processing/Speech, NLP and the Web (Lecture 10, 11–MT approaches) Pushpak Bhattacharyya CSE Dept., IIT Bombay 25 th Jan and 27 th Jan, 2011 Acknowledgement: parts are from Hansraj’s dual degree seminar presentation

2 Czeck-English data [nesu]“I carry” [ponese]“He will carry” [nese]“He carries” [nesou]“They carry” [yedu]“I drive” [plavou]“They swim”

3 To translate … I will carry. They drive. He swims. They will drive.

4 Hindi-English data [DhotA huM]“I carry” [DhoegA]“He will carry” [DhotA hAi]“He carries” [Dhote hAi]“They carry” [chalAtA huM]“I drive” [tErte hEM]“They swim”

5 Bangla-English data [bai]“I carry” [baibe]“He will carry” [bay]“He carries” [bay]“They carry” [chAlAi]“I drive” [sAMtrAy]“They swim”

6 MT Approaches words syntax semantics interlingua phrases words SOURCETARGET

7 Taxonomy MT Approaches Knowledge Based; Rule Based MT Data driven; Machine Learning Based Example Based MT (EBMT) Statistical MT Interlingua BasedTransfer Based

8 Motivation MT: NLP Complete NLP: AI complete AI: CS complete How will the world be different when the language barrier disappears? Volume of text required to be translated currently exceeds translators’ capacity (demand outstrips supply). Solution: automation (the only solution) Many machine translation techniques Which approach is better for Hindi-English MT

9 Interlingual representation: complete disambiguation Washington voted Washington to power Washingto n power action place capability … person agent object goal

10 Kinds of disambiguation needed for a complete and correct interlingua graph N: Name P: POS A: Attachment S: Sense C: Co-reference R: Semantic Role

11 Target Sentence Generation from interlingua Lexical Transfer Target Sentence Generation Syntax Planning Morphological Synthesis (Word/Phrase Translation ) (Word form Generation) (Sequence)

12 Role of function word Washington voted Washington to power. Washington agent ne Washington object ko sattaa goal ke liye chunaa Vote- chunna Power- sattaa

13 Statistical Machine Translation (SMT) Data driven approach Goal is to find out the English sentence e given foreign language sentence f whose p(e|f) is maximum. Translations are generated on the basis of statistical model Parameters are estimated using bilingual parallel corpora

14 SMT: Language Model To detect good English sentences Probability of an English sentence s 1 s 2 …… s n can be written as Pr(s 1 s 2 …… s n ) = Pr(s 1 ) * Pr(s 2 |s 1 ) *... * Pr(s n |s 1 s 2... s n-1 ) Here Pr(s n |s 1 s 2... s n-1 ) is the probability that word s n follows word string s 1 s 2... s n-1. N-gram model probability Trigram model probability calculation

15 SMT: Translation Model P(f|e): Probability of some f given hypothesis English translation e How to assign the values to p(e|f) ? Sentences are infinite, not possible to find pair(e,f) for all sentences Introduce a hidden variable a, that represents alignments between the individual words in the sentence pair Sentence level Word level

16 Alignment If the string, e= e 1 l = e 1 e 2 …e l, has l words, and the string, f= f 1 m =f 1 f 2...f m, has m words, then the alignment, a, can be represented by a series, a 1 m = a 1 a 2...a m, of m values, each between 0 and l such that if the word in position j of the f-string is connected to the word in position i of the e-string, then a j = i, and if it is not connected to any English word, then a j = O

17 Example of alignment English: Ram went to school Hindi: Raama paathashaalaa gayaa Ram wenttoschool Raamapaathashaalaagayaa

18 Translation Model: Exact expression Five models for estimating parameters in the expression [2] Model-1, Model-2, Model-3, Model-4, Model-5 Choose alignment given e and m Choose the identity of foreign word given e, m, a Choose the length of foreign language string given e

19 Proof of Translation Model: Exact expression m is fixed for a particular f, hence ; marginalization

20 Model-1 Simplest model Assumptions Pr(m|e) is independent of m and e and is equal to ε Alignment of foreign language words (FLWs) depends only on length of English sentence = ( l +1) -1 l is the length of English sentence The likelihood function will be Maximize the likelihood function constrained to

21 Model-1: Parameter estimation Using Lagrange multiplier for constrained maximization, the solution for model-1 parameters λ e : normalization constant; c(f|e; f,e) expected count; δ(f,f j ) is 1 if f & f j are same, zero otherwise. Estimate t(f|e) using Expectation Maximization (EM) procedure

22 Model-2 Alignment of FLW to an Eng word depends on its position The likelihood function is Model-1 & 2 Model-1 is special case of model-2 where To instantiate the model-2 parameters, use parameter estimated in model-1

23 Model-3 Fertility: Number of FLWs to which an Eng word is connected in a randomly selected alignment Tablet: List of FLWs connected to an Eng word Tableau: The collection of tablets The alignment process for each English word Begin Decide the fertility of the word Get a list of French words to connect to the word End Permute words in tableau to generate f

24 Model-3: Example English Sentence (e) = Annual inflation rises to 11.42% Step-1: Deciding fertilities(F) e = Annual inflation rises to 11.42% F = Annual inflation inflation inflation rises rises rises to 11.42%

25 Model-3: Example English Sentence (e) = Annual inflation rises to 11.42% Step-2: Translation to FLWs(T) e = Annual inflation rises to 11.42% F = Annual inflation inflation inflation rises rises rises to 11.42% T= वार्षिक मुद्रास्फीति की दर बढ गई है तक 11.42%

26 Model-3: Example English Sentence (e) = Annual inflation rises to 11.42% Step-3: Reordering FLWs(R) e = Annual inflation rises to 11.42% F = Annual inflation inflation inflation rises rises rises to 11.42% T = वार्षिक मुद्रास्फीति की दर बढ गई है तक 11.42% R = वार्षिक मुद्रास्फीति की दर 11.42% तक बढ गई है Values fPr, T, R calculated using the formulas obtained in model-3 [2]

27 Model-4 & 5 Model-3: Every word is moved independently Model-4: Consider phrases (cept) in a sentence Distortion probability is replaced by A parameter for head of the each cept A parameter for the remaining part of the cept Deficiency in model-3 & 4 In distortion probability Model-5 removes the deficiency Avoid unavailable positions Introduces a new variable for the positions

28 Example Based Machine Translation (EBMT) Basic idea: translate a sentence by using the closest match in parallel data Inspired by human analogical thinking

29 Issues Related to Examples in Corpora Granularity of examples Parallel text should be aligned at the subsentence level Number of examples Suitability of examples (i) Columbus discovered America (ii) America was discovered by Columbus (a) Time flies like an arrow (b) Time flies like an arrow How examples should be stored? Annotated tree structure Generalized examples “Rajesh will reach Mumbai by 10:00 pm”->”P will reach D by T”

30 Annotated Tree Structure: example Fully annotated tree with explicit links

31 EBMT: Matching and Retrieval ( 1/2 ) System must be able to recognize the similarity and differences b/w the input and stored examples String based matching: Longest common subsequence Takes word similarity into account while sense disabiguation

32 EBMT: Matching and Retrieval ( 2/2 ) Angle of similarity: Trigonometric similarity measure based on relative length & relative contents (x). Select ‘Symbol’ in the Insert menu. (y). Select ‘Symbol’ in the Insert menu to enter a character from the symbol set. (z). Select ‘Paste’ in the Edit menu. (w). Select ‘Paste’ in the Edit menu to enter some text from the clip board. θ xy : the qualitative difference between sentence x and y δ(x,y): the difference between size of x and y,

33 EBMT: Adaptation & Recombination Adaptation Extracting appropriate fragments from the matched translation The boy entered the house-> लड़के ने कमरे में प्रवेश किया I saw a tiger -> मैंने एक चीता देखा The boy eats his breakfast -> लड़के ने उसका नास्ता खाया था I saw the boy -> मैंने लड़के को देखा था Boundary Friction Retrieved translations do not fit the syntactic context I saw the boy -> * मैंने लड़के ने देखा था Recombine fragments into target text SMT “language model” can be used

34 Interlingua Based MT Interlingua "between languages“ SL text converted into a language-independent or 'universal' abstract representation then transform into several TL

35 Universal Networking Language ( UNL ) UNL is an example of interlingua Represents information sentence by sentence UNL is composed of Universal words Relations Example: “I gave him a book” {unl} agt ( i ) obj ( ) gol ( he ) {/unl}

36 Issues related to interlingua Interlingua must Capture the knowledge in text precisely and accurately Handle cross language divergence Divergence between Hindi-English language Constituent order divergence Null subject divergence जा रहा हु == * am going (I am going) Conflational divergence जीम ने जोहन को छुरे से मारा == Jim stabbed John Promotional divergence The play is on == खेल चल रहा है

37 Benefits & Shortcomings ( 1/3 ) Statistical Machine translation “Every time I fire a linguist, my system’s performance improves” (Brown et al. 1988) Pros No linguistic knowledge is required Great deal of natural language in machine readable text Loose dependencies b/w languages can be modeled better Cons Probability of rare words can’t be trusted Not good for idioms, jokes, compound words, text having hidden meaning Selection of correct morphological word is difficult

38 Benefits & Shortcomings ( 2/3 ) Example Based MT Pros Perfect translation of a sentence if very similar one found in example sentences No need to bother about previously translated sentences Cons Fails if no match found in corpora Problem at points of example concatenation in recombination step

39 Benefits & Shortcomings ( 3/3 ) Interlingua based MT Pros Add a new language and get all-ways translation to all previously added languages Monolingual lingual development team Economical in situation where translation among multiple languages is used Cons “Meaning” is arbitrarily deep. At what level of detail do we stop? Human development time

40 Translation is Ubiquitous Between Languages Delhi is the capital of India दिल्ली भारत की राजधानी है Between dialects Example next slide Between registers My “mom” not well. My “mother” is unwell (in a leave application)

41 Between dialects (1/3) Lage Raho Munnabhai: an excellent example Scene: Munnabhai (Sanjay Dutt) is Prof. Murli Prasad Sharma being interviewed with some citizens asking questions in presence of Jahnavi (Vidya Baalan) Question by citizen: प्रोफेसर साब, पार्क में एक नौजवान पत्थर उठा के बापू के मूर्ति पर मारा और उसका एक हाथ टूटा दिया. मेरे समझ में नही आया में उस नौजवान को केसे समझाऊ.

42 Between dialects (2/3) Bapu from behind invisible to others: उस के हाथ में एक पत्थर देकर कहना चाहिए था बापू का इस पुतला गिरा दो Munnabhai उस का हाथ में एक पत्थर देने का और कहनेका की बापू का इस पुतला गिरा दो Bapu इस देश में मेरे जितना भी पुतला है सब गिरा दो Munnabhai ई full country में मेरा जितना भी पुतला है सब गिरा दो Bapu हर इमारत हर चौराहे हर मार्ग से मेरा नाम मिटा दो Munnabhai हर बिल्डिंग हर नोट वोट रोड से मेरा नाम मिटा दो

43 Between dialects (3/3) Bapu मेरे हर तसबीर को दीवार से हठा दो Munnabhai मेरे जितना भी तसबीर दीवार पे लटकेला है ना, उसको निकाल के फेक दो Bapu अगर कही रखना है तो अपने दिलो में रखो Munnabhai क्या है की कही रखना छे तो, अपने दिल में रखो ना, समझा क्या, इधर heart में heart में !

44 Comparison b/w SMT, EBMT, Interlingua PropertyExample Based MTStatistical MTInterlingua based MT Parallel Corpora Yes No DictionaryYesNoYes Transfer RulesNo Yes ParserYesNoYes Semantic analysis No Yes Data driven incremental improvement Yes No Translation speed Slow Fast Language Dependency No Yes Intermediate meaning representation No Yes (universal representation)

45 References (1/2) 1.P. Brown, S. Della Pietra, V. Della Pietra, and R. Mercer. The mathematics of statistical machine translation: parameter estimation. Computational Linguistics, 19(2), (1993) 2.Makoto Nagao. A framework of a mechanical translation between Japanese and English by analogy principle, in A. Elithorn and R. Banerji: Artificial and Human Intelligence. Elsevier Science Publishers. (1984). 3.Somers H. Review Article: Example based Machine Translation. Machine Translation, Volume 14, Number 2, pp (45). (June 1999) 4.D. Turcato, F. Popowich. What is Example-Based Machine Translation? In M. Carl and A. Way (eds). Recent Advances of EBMT. Kluwer Adacemic Publishers, Dordrecht. Note, revised version of Workshop Paper. (2003)

46 References (2/2) 5.Dave S., Parikh J. and Bhattacharyya. Interlingua Based English Hindi Machine Translation and Language Divergence. P. Journal of Machine Translation, Volume 17. (2002) 6.Adam L. Berger, Stephen A. Della Pietra Y, Vincent J. Della Pietra Y. A maximum entropy approach to natural language processing. Computational Linguistics, (22-1), (March 1996). 7.Jason Baldridge, Tom Morton, and Gann Bierner. The opennlp.maxent package: POS tagger, end of sentence detector, tokenizer, name finder. version (Oct. 2005) 8.Universal Networking Language (UNL) Specifications. UNL Center of UNDL Foundation. URL: 7 June 2005.

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