1. The current status of Chinese-English EBMT research -where are we now Joy, Ralf Brown, Robert Frederking, Erik Peterson Aug 2001

2. Language Technologies Institute School of Computer Science, Carnegie Mellon University 2 Overview of Ch-En EBMT Adapting EBMT to Chinese –Segmentation of Chinese Corpus used –Hong Kong legal code (from LDC) –Hong Kong news articles (from LDC) In this project: –Robert Frederking, Ralf Brown, Joy, Erik Peterson, Stephan Vogel, Alon Lavie, Lori Levin,

3. Language Technologies Institute School of Computer Science, Carnegie Mellon University 3 Corpus Statistics Hong Kong Legal Code: Chinese: 23 MB English: 37.8 MB Hong Kong News (After cleaning): 7622 Documents Dev-test: Size: 1,331,915 byte, 4,992 sentence pairs Final-test: Size: 1,329,764 byte, 4,866 sentence pairs Training: Size: 25,720,755 byte, 95,752 sentence pairs Corpus Cleaning –Converted from Big5 to GB –Divided into Training set (90%), Dev-test (5%) and test set (5%) –Sentence level alignment, using Church & Gale Method (by ISI) –Cleaned –Convert two-byte Chinese characters to their cognates

4. Language Technologies Institute School of Computer Science, Carnegie Mellon University 4 Chinese Segmentation There are no spaces between Chinese words in written Chinese. The segmentation problem: Given a sentence with no spaces, break it into words. Definition of Chinese word is vague.

5. Language Technologies Institute School of Computer Science, Carnegie Mellon University 5 Our Definition of Words/Phrases/Terms Chinese Characters –The smallest unit in written Chinese is a character, which is represented by 2 bytes in GB-2312 code. Chinese Words –A word in natural language is the smallest reusable unit which can be used in isolation. Chinese Phrases –We define a Chinese phrase as a sequence of Chinese words. For each word in the phrase, the meaning of this word is the same as the meaning when the word appears by itself. Terms –A term is a meaningful constituent. It can be either a word or a phrase.

6. Language Technologies Institute School of Computer Science, Carnegie Mellon University 6 Complicated Constructions Transliterated foreign words and names Abbreviations Chinese Names Chinese Numbers

7. Language Technologies Institute School of Computer Science, Carnegie Mellon University 7 Segmenter Approaches –Statistical approaches: Idea: Building collocation models for Chinese characters, such as first-order HMM. Place the space at the place where two characters rarely co-occur. Cons: –Data sparseness –Cross boundary

8. Language Technologies Institute School of Computer Science, Carnegie Mellon University 8 Segmenter (2) –Dictionary-based approaches Idea: Use a dictionary to find the words in the sentence Forward maximum match / backward maximum match/ or both direction Cons: –The size and quality of the dictionary used are of great importance: New words, Named-entity –Maximum (greedy) match may cause mis-segmentations

9. Language Technologies Institute School of Computer Science, Carnegie Mellon University 9 Segmenter (3) –A combination of dictionary and linguistic knowledge Ideas: Using morphology, POS, grammar and heuristics to aid disambiguation Pros: high accuracy (possible) Cons: –Require a dictionary with POS and word-frequency –Computationally expensive

10. Language Technologies Institute School of Computer Science, Carnegie Mellon University 10 Segmenter (4) We first used LDC’s segmenter Currently we are using a forward/backward maximum match segmenter for baseline. The word frequency dictionary is from LDC The word frequency dictionary from LDC: 43,959 entries For HLT 2001, we augmented the frequency dictionary with new words found from the corpus by statistical method

11. Language Technologies Institute School of Computer Science, Carnegie Mellon University 11 Two-threshold method Two-threshold for tokenization (finding new words from the corpus) : for MT Summit VIII

12. Language Technologies Institute School of Computer Science, Carnegie Mellon University 12 For PI Meeting Baseline System –Using LDC’s frequency word dictionary Full System –Tokenize new words from the pre-segmented corpus using two- threshold method, augment the frequency dictionary with new words to re-segment the corpus –Bracket English –Using feedback from statDict to adjust segmentation/bracketing Baseline + Named-Entity –Named-entity tagger by Erik Peterson Multi-corpora System –Cluster the documents into sub-corpora according to their topics

13. Language Technologies Institute School of Computer Science, Carnegie Mellon University 13 Evaluation Issues Automatic Measures –EBMT Source Match –EBMT Source Coverage –EBMT Target Coverage –MEMT (EBMT+DICT) Unigram Coverage –MEMT (EBMT+DICT) PER

14. Language Technologies Institute School of Computer Science, Carnegie Mellon University 14 Evaluation Issues (2) Human Evaluations –4-5 graders each time –6 categories

15. Language Technologies Institute School of Computer Science, Carnegie Mellon University 15 After PI Meeting (0) Study of results reported in PI meeting (http://pizza.is.cs.cmu.edu/research/internal/ebmt/tokenLen/index.htm) –The quality of Named-Entity (Cleaned by Erik) –Performance difference of EBMT while changing the average length of Chinese word token (by changing segmentation) –How to evaluate the performance of the system Experiment of G-EBMT –Word clustering

16. Language Technologies Institute School of Computer Science, Carnegie Mellon University 16 After PI Meeting (1) Changing the average length of Chinese token –No bracket on English –Use a subset of LDC’s frequency dictionary for segmentation –Study the performance of EBMT system on different average Chinese token length

17. Language Technologies Institute School of Computer Science, Carnegie Mellon University 17 After PI Meeting (2) Avg. Token Len. Vs. PER

18. Language Technologies Institute School of Computer Science, Carnegie Mellon University 18 After PI Meeting (3) Type-Token curve of Chinese and English

19. Language Technologies Institute School of Computer Science, Carnegie Mellon University 19 Future Research Plan Generalized EBMT –Word-clustering –Grammar Induction Using Machine Learning to optimize the parameters used in MEMT Better Alignment Model: Integrating segmentation, brackting and alignment

20. Language Technologies Institute School of Computer Science, Carnegie Mellon University 20 New Alignment Model (1) Using both monolingual and bilingual collocation information to segment and align corpus

21. Language Technologies Institute School of Computer Science, Carnegie Mellon University 21 References Tom Emerson, “Segmentation of Chinese Text”. In #38 Volume 12 Issue2 of MultiLingual Computing & Technology published by MultiLingual Computing, Inc. Ying Zhang, Ralf D. Brown, and Robert E. Frederking. "Adapting an Example-Based Translation System to Chinese". To appear in Proceedings of Human Language Technology Conference 2001 (HLT-2001). Ying Zhang, Ralf D. Brown, Robert E. Frederking and Alon Lavie. "Pre- processing of Bilingual Corpora for Mandarin-English EBMT". Accepted in MT Summit VIII (Santiago de Compostela, Spain, Sep. 2001)