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Stat-XFER: A General Framework for Search-based Syntax-driven MT

Stat-XFER: A General Framework for Search-based Syntax-driven MT. Alon Lavie Language Technologies Institute Carnegie Mellon University Joint work with: Erik Peterson, Alok Parlikar, Vamshi Ambati, Abhaya Agarwal, Greg Hanneman, Kevin Gimpel, Edmund Huber. Outline. Context and Rationale

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Stat-XFER: A General Framework for Search-based Syntax-driven MT

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  1. Stat-XFER: A General Framework for Search-based Syntax-driven MT Alon Lavie Language Technologies Institute Carnegie Mellon University Joint work with: Erik Peterson, Alok Parlikar, Vamshi Ambati, Abhaya Agarwal, Greg Hanneman, Kevin Gimpel, Edmund Huber

  2. Outline • Context and Rationale • CMU Statistical Transfer MT Framework • Automatic Acquisition of Syntax-based MT Resources • Chinese-to-English System • Urdu-to-English System • Open Research Challenges • Conclusions NIST MT-08: CMU Stat-XFER

  3. Rule-based vs. Statistical MT • Traditional Rule-based MT: • Expressive and linguistically-rich formalisms capable of describing complex mappings between the two languages • Accurate “clean” resources • Everything constructed manually by experts • Main challenge: obtaining broad coverage • Phrase-based Statistical MT: • Learn word and phrase correspondences automatically from large volumes of parallel data • Search-based “decoding” framework: • Models propose many alternative translations • Effective search algorithms find the “best” translation • Main challenge: obtaining high translation accuracy NIST MT-08: CMU Stat-XFER

  4. Research Goals • Long-term research agenda (since 2000) focused on developing a unified framework for MT that addresses the core fundamental weaknesses of previous approaches: • Representation – explore richer formalisms that can capture complex divergences between languages • Ability to handle morphologically complex languages • Methods for automatically acquiring MT resources from available data and combining them with manual resources • Ability to address both rich and poor resource scenarios • Focus has been on low-resource scenarios, scaling up to resource-rich scenarios in the past year • Main research funding sources: NSF (AVENUE and LETRAS projects) and DARPA (GALE) NIST MT-08: CMU Stat-XFER

  5. CMU Statistical Transfer (Stat-XFER) MT Approach • Integrate the major strengths of rule-based and statistical MT within a common framework: • Linguistically rich formalism that can express complex and abstract compositional transfer rules • Rules can be written by human experts and also acquired automatically from data • Easy integration of morphological analyzers and generators • Word and syntactic-phrase correspondences can be automatically acquired from parallel text • Search-based decoding from statistical MT adapted to find the best translation within the search space: multi-feature scoring, beam-search, parameter optimization, etc. • Framework suitable for both resource-rich and resource-poor language scenarios NIST MT-08: CMU Stat-XFER

  6. Stat-XFER MT Approach Semantic Analysis Sentence Planning Interlingua Syntactic Parsing Transfer Rules Text Generation Statistical-XFER Source (e.g. Quechua) Target (e.g. English) Direct: SMT, EBMT NIST MT-08: CMU Stat-XFER

  7. Source Input בשורה הבאה Preprocessing Morphology Transfer Rules Language Model + Additional Features {NP1,3} NP1::NP1 [NP1 "H" ADJ] -> [ADJ NP1] ((X3::Y1) (X1::Y2) ((X1 def) = +) ((X1 status) =c absolute) ((X1 num) = (X3 num)) ((X1 gen) = (X3 gen)) (X0 = X1)) Transfer Engine Translation Lexicon Decoder N::N |: ["$WR"] -> ["BULL"] ((X1::Y1) ((X0 NUM) = s) ((Y0 lex) = "BULL")) N::N |: ["$WRH"] -> ["LINE"] ((X1::Y1) ((X0 NUM) = s) ((Y0 lex) = "LINE")) Translation Output Lattice (0 1 "IN" @PREP) (1 1 "THE" @DET) (2 2 "LINE" @N) (1 2 "THE LINE" @NP) (0 2 "IN LINE" @PP) (0 4 "IN THE NEXT LINE" @PP) English Output in the next line

  8. Type information Part-of-speech/constituent information Alignments x-side constraints y-side constraints xy-constraints, e.g. ((Y1 AGR) = (X1 AGR)) Transfer Rule Formalism ;SL: the old man, TL: ha-ish ha-zaqen NP::NP [DET ADJ N] -> [DET N DET ADJ] ( (X1::Y1) (X1::Y3) (X2::Y4) (X3::Y2) ((X1 AGR) = *3-SING) ((X1 DEF = *DEF) ((X3 AGR) = *3-SING) ((X3 COUNT) = +) ((Y1 DEF) = *DEF) ((Y3 DEF) = *DEF) ((Y2 AGR) = *3-SING) ((Y2 GENDER) = (Y4 GENDER)) ) NIST MT-08: CMU Stat-XFER

  9. Translation Lexicon: Examples PRO::PRO |: ["ANI"] -> ["I"] ( (X1::Y1) ((X0 per) = 1) ((X0 num) = s) ((X0 case) = nom) ) PRO::PRO |: ["ATH"] -> ["you"] ( (X1::Y1) ((X0 per) = 2) ((X0 num) = s) ((X0 gen) = m) ((X0 case) = nom) ) N::N |: ["$&H"] -> ["HOUR"] ( (X1::Y1) ((X0 NUM) = s) ((Y0 NUM) = s) ((Y0 lex) = "HOUR") ) N::N |: ["$&H"] -> ["hours"] ( (X1::Y1) ((Y0 NUM) = p) ((X0 NUM) = p) ((Y0 lex) = "HOUR") ) NIST MT-08: CMU Stat-XFER

  10. Hebrew Transfer GrammarExample Rules {NP1,2} ;;SL: $MLH ADWMH ;;TL: A RED DRESS NP1::NP1 [NP1 ADJ] -> [ADJ NP1] ( (X2::Y1) (X1::Y2) ((X1 def) = -) ((X1 status) =c absolute) ((X1 num) = (X2 num)) ((X1 gen) = (X2 gen)) (X0 = X1) ) {NP1,3} ;;SL: H $MLWT H ADWMWT ;;TL: THE RED DRESSES NP1::NP1 [NP1 "H" ADJ] -> [ADJ NP1] ( (X3::Y1) (X1::Y2) ((X1 def) = +) ((X1 status) =c absolute) ((X1 num) = (X3 num)) ((X1 gen) = (X3 gen)) (X0 = X1) ) NIST MT-08: CMU Stat-XFER

  11. The Transfer Engine • Input: source-language input sentence, or source-language confusion network • Output: lattice representing collection of translation fragments at all levels supported by transfer rules • Basic Algorithm: “bottom-up” integrated “parsing-transfer-generation” guided by the transfer rules • Start with translations of individual words and phrases from translation lexicon • Create translations of larger constituents by applying applicable transfer rules to previously created lattice entries • Beam-search controls the exponential combinatorics of the search-space, using multiple scoring features NIST MT-08: CMU Stat-XFER

  12. The Transfer Engine • Some Unique Features: • Works with either learned or manually-developed transfer grammars • Handles rules with or without unification constraints • Supports interfacing with servers for morphological analysis and generation • Can handle ambiguous source-word analyses and/or SL segmentations represented in the form of lattice structures NIST MT-08: CMU Stat-XFER

  13. XFER Output Lattice (28 28 "AND" -5.6988 "W" "(CONJ,0 'AND')") (29 29 "SINCE" -8.20817 "MAZ " "(ADVP,0 (ADV,5 'SINCE')) ") (29 29 "SINCE THEN" -12.0165 "MAZ " "(ADVP,0 (ADV,6 'SINCE THEN')) ") (29 29 "EVER SINCE" -12.5564 "MAZ " "(ADVP,0 (ADV,4 'EVER SINCE')) ") (30 30 "WORKED" -10.9913 "&BD " "(VERB,0 (V,11 'WORKED')) ") (30 30 "FUNCTIONED" -16.0023 "&BD " "(VERB,0 (V,10 'FUNCTIONED')) ") (30 30 "WORSHIPPED" -17.3393 "&BD " "(VERB,0 (V,12 'WORSHIPPED')) ") (30 30 "SERVED" -11.5161 "&BD " "(VERB,0 (V,14 'SERVED')) ") (30 30 "SLAVE" -13.9523 "&BD " "(NP0,0 (N,34 'SLAVE')) ") (30 30 "BONDSMAN" -18.0325 "&BD " "(NP0,0 (N,36 'BONDSMAN')) ") (30 30 "A SLAVE" -16.8671 "&BD " "(NP,1 (LITERAL 'A') (NP2,0 (NP1,0 (NP0,0 (N,34 'SLAVE')) ) ) ) ") (30 30 "A BONDSMAN" -21.0649 "&BD " "(NP,1 (LITERAL 'A') (NP2,0 (NP1,0 (NP0,0 (N,36 'BONDSMAN')) ) ) ) ") NIST MT-08: CMU Stat-XFER

  14. The Lattice Decoder • Simple Stack Decoder, similar in principle to simple Statistical MT decoders • Searches for best-scoring path of non-overlapping lattice arcs • No reordering during decoding • Scoring based on log-linear combination of scoring features, with weights trained using Minimum Error Rate Training (MERT) • Scoring components: • Statistical Language Model • Rule Scores (currently: freq-based relative likelihood) • Lexical Probabilities • Fragmentation: how many arcs to cover the entire translation? • Length Penalty: how far from expected target length? NIST MT-08: CMU Stat-XFER

  15. XFER Lattice Decoder 0 0 ON THE FOURTH DAY THE LION ATE THE RABBIT TO A MORNING MEAL Overall: -8.18323, Prob: -94.382, Rules: 0, Frag: 0.153846, Length: 0, Words: 13,13 235 < 0 8 -19.7602: B H IWM RBI&I (PP,0 (PREP,3 'ON')(NP,2 (LITERAL 'THE') (NP2,0 (NP1,1 (ADJ,2 (QUANT,0 'FOURTH'))(NP1,0 (NP0,1 (N,6 'DAY')))))))> 918 < 8 14 -46.2973: H ARIH AKL AT H $PN (S,2 (NP,2 (LITERAL 'THE') (NP2,0 (NP1,0 (NP0,1 (N,17 'LION')))))(VERB,0 (V,0 'ATE'))(NP,100 (NP,2 (LITERAL 'THE') (NP2,0 (NP1,0 (NP0,1 (N,24 'RABBIT')))))))> 584 < 14 17 -30.6607: L ARWXH BWQR (PP,0 (PREP,6 'TO')(NP,1 (LITERAL 'A') (NP2,0 (NP1,0 (NNP,3 (NP0,0 (N,32 'MORNING'))(NP0,0 (N,27 'MEAL')))))))> NIST MT-08: CMU Stat-XFER

  16. Stat-XFER MT Systems • General Stat-XFER framework under development for past seven years • Systems so far: • Chinese-to-English • Hebrew-to-English • Urdu-to-English • Hindi-to-English • Dutch-to-English • Mapudungun-to-Spanish • In progress or planned: • Arabic-to-English • Brazilian Portuguese-to-English • Native-Brazilian languages to Brazilian Portuguese • Hebrew-to-Arabic • Quechua-to-Spanish • Turkish-to-English NIST MT-08: CMU Stat-XFER

  17. MT Resource Acquisition in Resource-rich Scenarios • Scenario: Significant amounts of parallel-text at sentence-level are available • Parallel sentences can be word-aligned and parsed (at least on one side, ideally on both sides) • Goal: Acquire syntax-based broad-coverage translation lexicons and transfer rule grammars automatically from the data • Syntax-based translation lexicons: • Broad-coverage constituent-level translation equivalents at all levels of granularity • Can serve as the elementary building blocks for transfer trees constructed at runtime using the transfer rules NIST MT-08: CMU Stat-XFER

  18. Acquisition Process • Automatic Process for Extracting Syntax-driven Rules and Lexicons from sentence-parallel data: • Word-align the parallel corpus (GIZA++) • Parse the sentences independently for both languages • Run our new PFAConstituent Alignerover the parsed sentence pairs • Extract all aligned constituentsfrom the parallel trees • Extract all derived synchronous transfer rulesfrom the constituent-aligned parallel trees • Construct a “data-base”of all extracted parallel constituents and synchronous rules with their frequencies and model them statistically (assign them relative-likelihood probabilities) NIST MT-08: CMU Stat-XFER

  19. PFA Constituent Node Aligner • Input: a bilingual pair of parsed and word-aligned sentences • Goal: find all sub-sentential constituent alignments between the two trees which are translation equivalents of each other • Equivalence Constraint: a pair of constituents <S,T> are considered translation equivalents if: • All words in yield of <S> are aligned only to words in yield of <T> (and vice-versa) • If <S> has a sub-constituent <S1> that is aligned to <T1>, then <T1> must be a sub-constituent of <T> (and vice-versa) • Algorithm is a bottom-up process starting from word-level, marking nodes that satisfy the constraints NIST MT-08: CMU Stat-XFER

  20. PFA Node Alignment Algorithm Example • Words don’t have to align one-to-one • Constituent labels can be different in each language • Tree Structures can be highly divergent NIST MT-08: CMU Stat-XFER

  21. PFA Node Alignment Algorithm Example • Aligner uses a clever arithmetic manipulation to enforce equivalence constraints • Resulting aligned nodes are highlighted in figure NIST MT-08: CMU Stat-XFER

  22. PFA Node Alignment Algorithm Example • Extraction of Phrases: • Get the Yields of the aligned nodes and add them to a phrase table tagged with syntactic categories on both source and target sides • Example: • NP # NP :: • 澳洲 # Australia NIST MT-08: CMU Stat-XFER

  23. PFA Node Alignment Algorithm Example • All Phrases from this tree pair: • IP # S :: 澳洲 是 与 北韩 有 邦交 的 少数 国家 之一 。 # Australia is one of the few countries that have diplomatic relations with North Korea . • VP # VP :: 是 与 北韩 有 邦交 的 少数 国家 之一 # is one of the few countries that have diplomatic relations with North Korea • NP # NP :: 与 北韩 有 邦交 的 少数 国家 之一 # one of the few countries that have diplomatic relations with North Korea • VP # VP :: 与 北韩 有 邦交 # have diplomatic relations with North Korea • NP # NP :: 邦交 # diplomatic relations • NP # NP :: 北韩 # North Korea • NP # NP :: 澳洲 # Australia

  24. PFA Constituent Node Alignment Performance • Evaluation Data: Chinese-English Treebank • Parallel Chinese-English Treebank with manual word-alignments • 3342 Sentence Pairs • Created a “Gold Standard” constituent alignments using the manual word-alignments and treebank trees • Node Alignments: 39874 (About 12/tree pair) • NP to NP Alignments: 5427 • Manual inspection confirmed that the constituent alignments are quite accurate (P > 0.80, R > 0.70) • Evaluation: Run PFA Aligner with automatic word alignments on same data and compare with the “gold Standard” alignments NIST MT-08: CMU Stat-XFER

  25. PFA Constituent Node Alignment Performance • Viterbi word alignments from Chinese-English and reverse directions were merged using different algorithms • Tested the performance of Node-Alignment with each resulting alignment NIST MT-08: CMU Stat-XFER

  26. Transfer Rule Learning • Input: Constituent-aligned parallel trees • Idea: Aligned nodes act as possible decomposition points of the parallel trees • The sub-trees of any aligned pair of nodes can be further decomposed at lower-level aligned nodes, creating an inventory of synchronous “tree fragment” correspondences • We decompose only at the “highest” level possible • Synchronous “tree frags” can be converted into synchronous rules • Algorithm: • Find and extract all possible synchronous tree fragdecompositions from the node aligned trees • “Flatten” the tree frags into synchronous CFG rules NIST MT-08: CMU Stat-XFER

  27. Rule Extraction Algorithm • Flat Rule Creation: • Sample rule: • NP::NP [VP 北 CD 有 邦交 ] -> [one of the CD countries that VP] • ( • ;; Alignments • (X1::Y7) • (X3::Y4) • ) • Note: • Any one-to-one aligned words are elevated to Part-Of-Speech in flat rule. • Any non-aligned words on either source or target side remain lexicalized

  28. Rule Extraction Algorithm All rules extracted: VP::VP [VC NP] -> [VBZ NP] ( (*score* 0.5) ;; Alignments (X1::Y1) (X2::Y2) ) VP::VP [VC NP] -> [VBZ NP] ( (*score* 0.5) ;; Alignments (X1::Y1) (X2::Y2) ) NP::NP [NR] -> [NNP] ( (*score* 0.5) ;; Alignments (X1::Y1) (X2::Y2) ) VP::VP [北 NP VE NP] -> [ VBP NP with NP] ( (*score* 0.5) ;; Alignments (X2::Y4) (X3::Y1) (X4::Y2) ) All rules extracted: NP::NP [VP 北 CD 有 邦交 ] -> [one of the CD countries that VP] ( (*score* 0.5) ;; Alignments (X1::Y7) (X3::Y4) ) IP::S [ NP VP ] -> [NP VP ] ( (*score* 0.5) ;; Alignments (X1::Y1) (X2::Y2) ) NP::NP [ “北韩”] -> [“North” “Korea”] ( ;Many to one alignment is a phrase )

  29. Chinese-English System • Developed over past year under DARPA/GALE funding (within IBM-led “Rosetta” team) • Participated in recent NIST MT-08 Evaluation • Large-scale broad-coverage system • Integrates large manual resources with automatically extracted resources • Current performance-level is still inferior to state-of-the-art phrase-based systems NIST MT-08: CMU Stat-XFER

  30. Chinese-English System • Lexical Resources: • Manual Lexicons (base forms): • LDC, ADSO, Wiki • Total number of entries: 1.07 million • Automatically acquired from parallel data: • Approx 5 million sentences LDC/GALE data • Filtered down to phrases < 10 words in length • Full formed • Total number of entries: 2.67 million NIST MT-08: CMU Stat-XFER

  31. Chinese-English System • Transfer Rules: • 61 manually developed transfer rules • High-accuracy rules extracted from manually word-aligned parallel data NIST MT-08: CMU Stat-XFER

  32. Translation Example • SrcSent 3 澳洲是与北韩有邦交的少数国家之一。 • Gloss: Australia is with north korea have diplomatic relations DE few country world • Reference: Australia is one of the few countries that have diplomatic relations with North Korea. • Translation: Australia is one of the few countries that has diplomatic relations with north korea . • Overall: -5.77439, Prob: -2.58631, Rules: -0.66874, TransSGT: -2.58646, TransTGS: -1.52858, Frag: -0.0413927, Length: -0.127525, Words: 11,15 • ( 0 10 "Australia is one of the few countries that has diplomatic relations with north korea" -5.66505 "澳洲 是 与 北韩 有 邦交 的 少数 国家 之一 " "(S1,1124731 (S,1157857 (NP,2 (NB,1 (LDC_N,1267 'Australia') ) ) (VP,1046077 (MISC_V,1 'is') (NP,1077875 (LITERAL 'one') (LITERAL 'of') (NP,1045537 (NP,1017929 (NP,1 (LITERAL 'the') (NUMNB,2 (LDC_NUM,420 'few') (NB,1 (WIKI_N,62230 'countries') ) ) ) (LITERAL 'that') (VP,1021811 (LITERAL 'has') (FBIS_NP,11916 'diplomatic relations') ) ) (FBIS_PP,84791 'with north korea') ) ) ) ) ) ") • ( 10 11 "." -11.9549 "。" "(MISC_PUNC,20 '.')") NIST MT-08: CMU Stat-XFER

  33. Example: Syntactic Lexical Phrases • (LDC_N,1267 'Australia') • (WIKI_N,62230 'countries') • (FBIS_NP,11916 'diplomatic relations') • (FBIS_PP,84791 'with north korea') NIST MT-08: CMU Stat-XFER

  34. Example: Learned XFER Rules ;;SL::(2,4) 对 台 贸易 ;;TL::(3,5) trade to taiwan ;;Score::22 {NP,1045537} NP::NP [PP NP ] -> [NP PP ] ((*score* 0.916666666666667) (X2::Y1) (X1::Y2)) ;;SL::(2,7) 直接 提到 伟 哥 的 广告 ;;TL::(1,7) commercials that directly mention the name viagra ;;Score::5 {NP,1017929} NP::NP [VP "的" NP ] -> [NP "that" VP ] ((*score* 0.111111111111111) (X3::Y1) (X1::Y3)) ;;SL::(4,14) 有 一 至 多 个 高 新 技术 项目 或 产品 ;;TL::(3,14) has one or more new , high level technology projects or products ;;Score::4 {VP,1021811} VP::VP ["有" NP ] -> ["has" NP ] ((*score* 0.1) (X2::Y2)) NIST MT-08: CMU Stat-XFER

  35. Current Performance NIST MT-08: CMU Stat-XFER

  36. Urdu-to-English System • Primary condition did not allow parsing of parallel data  low-resource scenario: • Lexical resources: used provided LDC lexicon (tagged with POS), plus lexical entries acquired from word-aligning parallel-data • XFER rules: manually developed (48 rules) • Language Model built from English side of parallel data • Primary and Contrastive systems: • Cont-1: Built a phrase-based system using Moses • Primary: Used our multi-engine MT system to combine our XFER system with our Moses system • Cont-2: Our constrained Stat-XFER system only • Cont-3: Unconstrained version of Stat-XFER with just a large LM • Cont-4: MEMT of Stat-XFER (Cont-2) and Columbia’s constrained phrase-based system • Cont-5: similar to Cont-4, but with unconstrained systems NIST MT-08: CMU Stat-XFER

  37. Urdu-to-English System • Our official reported scores are incorrect due to a “one-off” bug half way through our output • Corrected scores, as reported by NIST NIST MT-08: CMU Stat-XFER

  38. Open Research Questions • Our large-scale Chinese-English system is still significantly behind phrase-based SMT. Why? • Feature set is not sufficiently discriminant? • Problems with the parsers for the two sides? • Weaker decoder? • Syntactic constituents don’t provide sufficient coverage? • Bugs and deficiencies in the underlying algorithms? • The ISI experience indicates that it may take a couple of years to catch up with and surpass the phrase-based systems • Significant engineering issues to improve speed and efficient runtime processing and improved search NIST MT-08: CMU Stat-XFER

  39. Open Research Questions • Immediate Research Issues: • Rule Learning: • Study effects of learning rules from manually vs automatically word aligned data • Study effects of parser accuracy on learned rules • Effective discriminant methods for modeling rule scores • Rule filtering strategies • Syntax-based LMs: • Our translations come out with a syntax-tree attached to them • Add a syntax-based LM feature that can discriminate between good and bad trees NIST MT-08: CMU Stat-XFER

  40. Conclusions • Stat-XFER is a promising general MT framework, suitable to a variety of MT scenarios and languages • Provides a complete solution for building end-to-end MT systems from parallel data, akin to phrase-based SMT systems (training, tuning, runtime system) • No open-source publicly available toolkits (yet), but we welcome further collaboration activities • Complex but highly interesting set of open research issues NIST MT-08: CMU Stat-XFER

  41. Questions? NIST MT-08: CMU Stat-XFER

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