13. Constantin Orasan (UoW) Natural Language Processing for Translation


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13. Constantin Orasan (UoW) Natural Language Processing for Translation

  1. 1. Natural Language Processing for Translation Constantin Orasan University of Wolverhampton, UK
  2. 2. Structure 1. How NLP can help machine translation 2. How information retrieval can help machine translation 3. View from the industry
  3. 3. MT is difficult  Language is ambiguous at all levels:  Lexical: bank, file, chair  Syntactic: John saw the man with the telescope.  Semantic: The rabbit is ready for lunch.  Discourse: John hid Bill’s car keys. He was drunk.  Pragmatics: You owe me £20.  It gets even more difficult when we start working in multilingual settings
  4. 4. Steps of processing       Tokenisation Morphology Syntax Lexical Semantics Discourse Pragmatics  NLP complexity increases this way! (and in general the accuracy of methods decreases)
  5. 5. General NLP frameworks  GATE (General Architecture for Text Engineering) http://gate.ac.uk framework written in Java:  First designed for information extraction tasks  Developed into a robust framework that offers processing a different linguistic levels in many languages  Provides wrappers for other tools such as Weka, LingPipe, etc.  NLTK: set of NLP modules written in Python with emphasis on teaching  Lingpipe (http://alias-i.com/lingpipe/) java toolkit for language processing  OpenNLP (http://opennlp.apache.org/) ML-based toolkit for language processing  … the list can go on
  6. 6. Vauquois triangle Direct translation Source text Source syntax Shallow (syntactic) transfer Target text Target syntax Source Deep (semantic) transfer Target semantics semantics Interlingua
  7. 7. NLP in SMT  Can benefit from linguistic information, but many of the existing models are largely data driven and do not incorporate much linguistic information  See the lecture on Statistical MT: Word, Phrase and Tree Based Models (overview) Khalil Sima'an (UvA) and Trevor Cohn (USFD)
  8. 8. NLP in EBMT  In many cases it requires some kind of linguistic information  See the lecture on Example Based Machine Translation Joseph van Genabith (DCU) and Kalil Sima'an (UvA)
  9. 9. NLP in TM  The existing TM solutions do not rely on much linguistic information  Second generation and third generations of TM rely on linguistic input  See the lecture on Translation Memories Ruslan Mitkov (UoW), Manuel Arcedillo (Hermes) and Juanjo Arevalillo (Hermes)
  10. 10. But there are many other ways in which we could improve the results of translation engines by incorporating linguistic information
  11. 11. Improve tokenisation  For European languages tokenisation is considered more or less a simple problem.  In non-segmented languages (such as many oriental ones), identification of tokens is extremely complex  Tokens do not have explicit boundaries (written directly adjacent to one another with no whitespace between them).  Practically all the characters can be one-character words in themselves, but they can also be joined together to form multicharacter words.  Even in segmented languages like English, identification of tokens can be difficult.
  12. 12. Tokenisation may not be so easy  Even in segmented languages like English where Tokens are usually separated by whitespaces and punctuation there are problem:  Abbreviations: when full stops follow abbreviations, they should be merged with the abbreviation to form one token (e.g. etc., yrs., Mr.)  Multiple strings separated by white space can in fact form one token (e.g., numerical expressions in French:1 200 000)  Hyphenation can be ambiguous:  Sometimes part of the word segment, e.g. self-assessment, F-16, forty-two,  Sometimes not, e.g. London-based  Additional challenges:  Numerical, special expressions (dates, measures, email addresses)  Language specific rules for contracting words and phrases (e.g. can’t, won’t vs. O'Brien: contain multiple tokens with no white spaces between)  Ambiguous punctuation (e.g. “.” in yrs., 05.11.08 )
  13. 13. Why tokenisation is important in MT?  It will influence any task that requires a dictionary/gazeeter lookup  Can influence how words are aligned  Abbreviations were shown to help SMT (Li and Yarowsky, 2008)  Translation of named entities (n.b. NER is seen as part of tokenization)
  14. 14. Abbreviations  Unseen abbreviations are treated as unknown words and left untranslated  Modern Chinese is a highly abbreviated language and 20% of sentences in an newspaper article contain an abbreviation  The way abbreviations are formed follows much more complex rules than English  Li and Yarowsky (2008) propose an unsupervised method for extracting relations between full-form phrases and their abbreviations
  15. 15. Li and Yarowsky (2008)  Step 1: Identification of English entities  Step 2: Translate the entities into Chinese using a baseline translator  Step 3: Full-abbreviations relations are extracted on the basis of co-occurrence in a Chinese monolingual corpus  Step 4: Translation induction for Chinese abbreviations  Step 5: Integration with the baseline translation system Evaluation shows that the results of the BLEU scores improve
  16. 16. Translation of named entities  Incorrect NE translation can seriously harm the quality of translation  There are 2 main sources of problems:  Ambiguity: NE normally are composed of words which can be translated in isolation  Sparsity: some named entities are very sparse  Integration of NEs into the translation model leads to various results ranging from significant improvements to low improvements and even negative impact
  17. 17. NE in SMT  The main approach in SMT is to determine the NEs in a text and translate them using an external model.  Then they are:  Used as the default translation (Li et al, 2009)  Added dynamically to compete with other translations (Turchi et al., 2012; Bouamor et al., 2012),  not used, and do not translate the original NE (Tinsley et al., 2012)  Nikoulina et al. (2012) propose replacing NEs with placeholders in order to reduce sparsity, in this way learning a better model
  18. 18. Nikoulina et al. (2012) 1. 2. 3. 4. The Named Entites are detected and replaced with placeholders to produce reduced sentences A reduced translation model is used to translate the reduced sentences An external NE translator is employed The translated NEs are reinserted in the reduced translations The disadvantage of the approach is that the framework is loosely dependent on the SMT task  a postprocessing step is applied to the output of the NER + a prediction model to determine which NEs can be safely translated
  19. 19. MT and semantics  Noted as problem for Machine Translation back in the late 1949’s (Weaver, 1949) A word can often only be translated if you know the specific sense intended  Bar-Hillel (1960) posed the following problem: Little John was looking for his toy box. Finally, he found it. The box was in the pen. John was very happy.  Is “pen” a writing instrument or an enclosure where children play or an enclosure for livestock?  …declared it unsolvable, and left the field of MT…
  20. 20. Lexical Divergence: many-to-many
  21. 21. Lexical Divergence: solution?  Domain specific dictionaries can improve the quality of translation in post-editing environments  Word sense disambiguation (WSD) is the ability to identify the meaning of words in context in a computational manner  WSD is seen as a more general solution to lexical divergence, but  WSD is an AI complete
  22. 22. Carpuat and Wu (2005)  SMT models only rely on local context to choose among lexical translation candidates  The assumption is that a dedicated WSD module can help the translation process  Use a baseline Chinese to English translation engine and a state-of-theart Chinese WSD  WSD incorporated in the decoder  WSD incorporated in a post-processor  Translation obtained using the English gloss of HowNet  WSD does not help a typical SMT, but this is mainly due to the fact that current SMT systems (2005) cannot take advantage of the sense information
  23. 23. Chan et al. (2007)  Successfully integrate WSD in Hiero, a state-of-the-art Chinese to English hierarchical phrase-based MT system  Introduce two additional features in the MT model at the decoding stage that take into consideration that some words were chosen by the WSD system  The improvement noticed is modest, but statistically significant  Carpuat and Wu (2007) find similar results, but instead of WSD they perform fully phrasal multi-word disambiguation and their disambiguation system is tightly integrated in the SMT engine
  24. 24. NLP in evaluation of MT  Evaluation metrics like BLUE treat any divergence from the reference translation as a mistake  Several alternative metrics were proposed to address this problem:  METEOR (Denkowski and Lavie, 2010) accounts for synonyms and paraphrases  Calculate meaning equivalence using bidirectional textual entailment (Pado et al., 2009)  Using semantic role labels (Gimenez and Marquez, 2007)  TINE (Rios et al., 2011) measures the similarity between sentences using shallow semantic representation
  25. 25. Other NLP applications which could be useful     Automatic terminology extraction Automatic extraction of ontologies Automatic compilation of (parallel/comparable) corpora Use of parallel corpora to train various systems
  26. 26. References  Bouamor, D., Semmar, N., and Zweigenbaum, P. (2012). Identifying multiword expressions in statistical machine translation. In Proceedings of LREC 2012.  Carpuat, M., & Wu, D. (2007). Improving Statistical Machine Translation Using Word Sense Disambiguation. In Proceedings of the 2007 Joint Conference on Empirical Methods in Natural Language Processing and Computational Natural Language Learning (pp. 61–72). Prague, Czech Republic. Retrieved from http://acl.ldc.upenn.edu/D/D07/D07-1007.pdf  Carpuat, M., & Wu, D. (2005). Word Sense Disambiguation vs. Statistical Machine Translation. Proceedings of the 43rd Annual Meeting of the ACL, (June), 387–394. Retrieved from http://acl.ldc.upenn.edu/P/P07/P071005.pdf  Chan, Y. S., Ng, H. T., & Chiang, D. (2007). Word Sense Disambiguation Improves Statistical Machine Translation. In Proceedings of the 45th Annual Meeting of the Association for Computational Linguistics (pp. 33–40).
  27. 27. References  Li, M., Zhang, J., Zhou, Y., and Chengqing, Z. (2009). The CASIA statistical machine translation system for IWSLT 2009. In Proceedings of IWSLT 2009  Li, Z., & Yarowsky, D. (2008). Unsupervised Translation Induction for Chinese Abbreviations using Monolingual Corpora. In Proceedings of ACL-08 (pp. 425 – 433). Columbus, Ohio, USA. Retrieved from http://aclweb.org/anthology//P/P08/P08-1049.pdf  Navigli, R. (2009). Word sense disambiguation. ACM Computing Surveys, 41(2), 1–69. doi:10.1145/1459352.1459355  Nikoulina, V., Sandor, A., & Dymetman, M. (2012). Hybrid Adaptation of Named Entity Recognition for Statistical Machine Translation. In Second ML4HMT Workshop (pp. 1–16).  Pado, S., Galley, M., Jurafsky, D., & Manning, C. (2009). Robust Machine Translation Evaluation with Entailment Features. In Proceedings of the 47th Annual Meeting of the ACL and the 4th IJCNLP of the AFNLP (pp. 297–305).
  28. 28. References  Rios, M.; Aziz, W.; Specia, L. (2011). TINE: A Metric to Assess MT Adequacy. In Proceedings of the 6th Workshop on Statistical Machine Translation (WMT-2011), July, Edinburgh, UK  Tinsley, J., Ceausu, A., and Zhang, J. (2012). PLUTO: automated solutions for patent translation. In EACL JointWorkshop on Exploitng Synergies between Information Retrieval andMachine Translation (ESIRMT) and Hybrid Approaches to Machine Translation (HyTra): Proceedings of the workshop, EACL 2012.  Turchi, M., Atkinson, M.,Wilcox, A., Crawley, B., Bucci, S., Steinberger, R., and Van der Goot, E. (2012). ONTS: "Optima" news translation system. In Proceedings of the Demonstrations at the 13th Conference of the European Chapter of the Association for Computational Linguistics.