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Grammatical Machine Translation Stefan Riezler & John Maxwell Overview 1. Introduction 2. Extracting F-Structure Snippets 3. Parsing-Transfer-Generation 4. Statistical Models and Training 5. Experimental Evaluation 6. Discussion Section 1: Introduction Introduction Recent approaches to SMT use • Phrase-based SMT • Syntactic knowledge Phrase-base SMT is great for • Local ordering • Short idiomatic expressions But not so good for • Learning LDDs • Generalising to unseen phrases that share non-overt linguistic info Statistical Parsers Statistical Parsers can provide information to • Resolve LDDs • Generalise to unseen phrases that share non-overt linguistic info Examples: • Xia & McCord 2004 • Collins et al. 2005 • Lin 2004 • Ding & Palmer 2005 • Quirk et al. 2005 Grammar-based Generation Could grammar-based generation be useful for MT? Quirk et al. 2005 • Simple statistical model outperforms grammar-base generator of Menezes & Richardson 2001 on BLEU score Charniak et al. 2003 • Parsing-based language modelling can improve grammaticality of translations while not improving BLEU score Perhaps BLEU score is not sufficient way to test for grammaticality. Further investigation needed Grammatical Machine Translation Aim: Investigate incorporating a grammar-based generator into a dependency-based SMT system The authors present: • A dependency-based SMT model • Statistical components that are modelled on phrase-based system of Koehn et al. 2003 Also used: • Component weights adjusted using MER training (Och 2003) • Grammar-based generator • N-gram and distortion models Section 2: Extracting F-Structure Snippets Extracting F-Structure Snippets SL and TL sentences of bilingual corpus parsed using LFG grammars For each English and German f-structure pair • The two f-structures that most preserve dependencies are selected • Many-to-many word alignments used to create many-to- many correspondences between the substructures • Correspondences are the basis for deciding what goes into the basic transfer rule Extracting F-Structure Snippets:Example Dafur bin ich zutiefst dankbar I have a deep appreciation for that <for that> <am> <I> <deepest> <thankful> Many-to-many bidirectional word alignment: Transfer Rule Extraction: Example From the aligned words we get the following substructure correspondences: Transfer Rule Extraction: Example From the correspondences two kinds of transfer rules are extracted: 1. Primitive Transfer Rules 2. Complex Transfer Rules Transfer Contiguity Constraint 1. Source and target f-structures are each connected. 2. F-structures in the transfer source can only be aligned with f-structures in the transfer target and vice versa. Transfer Rule Extraction: Example Primitive Rule 1: pred( X1, sein) pred( X1, have) subj( X1, X2) subj( X1, X2) xcomp( X1, X3) obj( X1, X3) Transfer Rule Extraction: Example Primitive Rule 2: pred( X1, ich) pred( X1, I) Transfer Rule Extraction: Example Primitive Rule 3: pred( X1, dafur) pred( X1, for) obj( X1, X2) pred( X2, that) Transfer Rule Extraction: Example Primitive Rule 4: pred( X1, dankbar) pred( X1, appreciation) adj( X1, X2) spec( X1, X2) in_set( X3, X2) pred( X2, a) pred(X3, zutiefst) adj( X1, X3) in_set( X4, X3) pred( X4, deep) Transfer Rule Extraction: Example Complex Transfer Rules • primitive transfer rules that are adjacent in f-structure combined to form more complex rules Example (rules 1 & 2 above): pred( X1, sein) pred( X1, have) subj( X1, X2) subj( X1, X2) pred( X2, ich) pred( X2, I) xcomp( X1, X3) obj( X1, X3) In the worst case, there can be an exponential number of combinations of primitive transfer rules, the number of primitive rules used to form a complex rule is restricted to 3 – causing the no. of transfer rules taken to be O(n2) in the worst case. Section 3: Parsing-Transfer-Generation Parsing • LFG grammars used to parse source and target text • FRAGMENT grammar is used to augment standard grammar increasing robustness • Correct parse determined by fewest chunk method Transfer • Rules applied to source f-structure non- deterministically and in parallel • Each fact of German f-structure translated by exactly one transfer rule • Default rule included that allows any fact to be translated as itself • Chart used to encode translations • Beam search decoding used to select the most probable translations Generation Method of generation has to be fault tolerant • Transfer system can be given a fragmentary parse as input • Transfer system can output an non-valid f- structure • Unknown predicates – Default morphology used to inflect source stem for English • Unknown structures – Default grammar used that allows any attribute to be generated in any order with any category Section 4: Statistical Models & Training Statistical Components Modelled on statistical components of Pharaoh Paraoh integrates 8 statistical models 1. Relative frequency of phrase translations in source-to- target 2. Relative frequency of phrase translations in target-to- source 3. Lexical weighting in source-to-target 4. Lexical weighting in target-to-source 5. Phrase count 6. Language model probability 7. Word count 8. Distortion probability Statistical Components Following statistics for each translation: 1. Log-probability of source-to-target transfer rules, where the probability r(e|f) of a rule that transfers source snippet f into target snippet e is estimated by the relative frequency 2. Log-probability of target-to-source rules Statistical Components 3. Log-probability of lexical translations from source to target snippets, estimated from Viterbi alignments â between source word positions i = 1, …, n and target word positions j = 1, …, m for stems fi and ej in snippets f and e with relative word translation frequencies t(ej|fi) 4. Log-probability of lexical translations from target-to- source snippets Statistical Components 5. Number of transfer rule 6. Number of transfer rules with frequency 1 7. Number of default transfer rules 8. Log-probability of strings of predicates from root to frontier of target f-structure, estimated from predicate trigrams of English 9. Number of predicates in target language 10. Number of constituent movements during generation based on the original order of the head predicates of the constituents (for example, AP BP CP counts as two movements since the head predicate of CP moved from first to third position) Statistical Components 11. Number of generation repairs 12. Log-probability of target string as computed by trigram language model 13. Number of words in target string • 1 – 10 are used to choose the most probable parse from the transfer chart • 1 – 7 are are tests on source and target f-structure snippets related via transfer rules • 8 -10 are language model and distortion features on the target c- and f-structures • 11 – 13 are computed on the strings that are generated from the target f-structure The statistics are combined into a log-linear model whose parameters are adjusted by minimum error rate training. Section 5: ExperimentalEvaluation Experimental Evaluation • Europarl German to English • Sents of length 5 – 15 words Training set: 163,141 sents Development set: 1,967 sents Test set: 1,755 sents (same as Koehn et al 2003) • Bidirectional word alignment created from word alignment of IBM model 4 as implemented by Giza++ (Och et al. 1999) • Grammars achieve 100% coverage on unseen data – 80% as full parses – 20% as fragment parses • 700,000 transfer rules extracted • For language modelling trigram model of Stolcke 2002 is used Experimental Evaluation For translating the test set • 1 parse for each German sentence was used • 10 transferred f-structures • 1,000 generated strings for each transferred f- structure • Most probable target f-structure is gotten by a beam search on the transfer chart using features 1-10 above, with a beam size of 20. • Features 11-13 are computed on the strings that are generated Experimental Evaluation • Promising results for examples that are in-coverage of LFG grammars • However, back-off to robustness techniques for parsing and generation results in loss of translation quality Rule Extraction Problems • 20% of the parses are fragmental • Errors occur in rule extraction process resulting in ill-formed transfer rules Parsing-Transfer-Generation Problems • Parsing errors errors in transfer generation errors • In-coverage disambiguation errors in parsing and transfer suboptimal translation Experimental Evaluation • Despite use of minimum error rate training and n-gram language models, the system cannot be used to maximize n-gram scores on reference translations in the same way as phrase-based systems since statistical ordering models are employed in the framework after generation • This gives preference to grammaticality over similarity to reference translations Conclusion • SMT model that marries phrase-based SMT with traditional grammar-based MT • NIST measure showed that results achieved are comparable with phrase-based SMT system of Koehn et al 2003 for in-coverage examples • Manual evaluation showed significant improvements in both grammaticality and translational adequacy for in-coverage examples Conclusion • Determinable with this system whether or not a source sentence is in-coverage • Possibility for hybrid system that achieves improved grammaticality at state-of-the-art translation quality Future Work: • Improvement of translation of in-coverage source sentences e.g. stochastic generation • Apply system to other language pairs and data sets References Miriam Butt, Dyvik Helge, Tracy King, Hiroshi Masuichi and Christian Rohrer. 2002 The Parallel Grammar Project. Eugene Charniak, Kevin Knight and Kenji Yamada. 2003 Syntax-based Language Models for Statistical Machine Translation. Michael Collins, PhilippKoehn and Ivona Kucerova. 2005 Clause Restructuring for Statistical Machine Translation. Philipp Koehn, Franz Och and Daniel Marcu. 2003 Statistical Phrase-based Translation. Philipp Koehn. 2004 Pharaoh: a beam search decoder for phrase-based statistical machine translation Arul Menezes and Stephen Richardson. 2001 A best-first alignment for automatic extraction of transfer mappings from bilingual corpora. Franz Och, Christoph Tillmann and Ney Hermann. 1999 Improved Alignment Models for Statistical Machine Translation. Franz Och. 2003 Minimum error rate training in statistical machine translation. Kishore Papineni, Salim Roukos, Todd Ward and Wei-Jing Zhu. 2002 BLEU: a method for automatic evaluation of machine translation. Stefan Riezler, Tracy King, Ronald Kaplan, Richard Crouch, John Maxwell and Mark Johnson. 2002 Parsing the Wall Street Journal using LFG and Discriminative Estimation Techniques Stefan Riezler and John Maxwell. 2006 Grammatical Machine Translation. Fei Xia and Michael McCord. 2004 Improving a statistical MT system with automatically learned rewrite patterns
"Grammatical Machine Translation"