1. Collocation Extraction Using Monolingual Word Alignment Method Zhanyi Liu, Haifeng Wang, Hua Wu, Sheng Li EMNLP 2009

2. Collocation • Two words • Consecutive ("by accident") • Interrupted ("take ... advice") • Other examples • Proper noun ("New York") • Compound nouns ("ice cream") • Correlative conjunction ("either ... or")

3. Previous Works • Co-occurring word pairs • Word pairs in a given window size • Association measures • Frequency, log-likelihood, mutual information ... • Disadvantage • Long-span collocation • "either ... or", "because ... so" • Limited by window size • False collocation • Any word pairs in window size

4. Monolingual Word Alignment • Bilingual word alignment (BWA) • Source-target sentence pairs • Monolingual Word Alignment (MWA) • Source-source sentence pairs • Replicate the corpus

5. Monolingual Word Alignment (2) Bilingual Monolingual A word never collocates with itself

6. MWA Model • Sentence with l words S ={w1,...,wl} • Alignment A = {(i,ai) | i∈[1,l]} A = {(2,3), (3,2), (4,7), (6,7)...}

7. MWA Model (2) • Adapt IBM Model 3 to MWA • EM training algorithm, produce 3 probability • Word collocation probability • Position collocation probability • d(4|7,12) • Prob that 4th collocates with 7th word in a 12-word sentence • Fertility probability • Prob that wi is collocate with Φi words

8. Collocation Extraction • Extract and rank. Filter when freq(wi,wj)<5 • Symmetric assumption • (wi, wj) = (wj, wi)

9. Initial Experiment • Chinese • Training data • LDC2007T03 Tagged Chinese Giga Word • Xinhua portion, 28M words • Gold set • Handcrafted collocation dictionaries • 56888 collocations

10. Initial Experiment (2) • Precision • Baseline • Frequency, log-likelihood, mutual information • Log-likelihood achieves the best performance

11. Initial Experiment (3) • Observation • Precision is low • Small gold set (57K/200K = 28%) • Low precision when N < 20K

12. Observation • Frequency vs. Probability vs. Precision • Precision curve • Lower freq --> lower precision • Alignment probability curve • Lower freq --> higher probability

13. Observation (2) • Conclusion • What causes lower precision of top 20K? • Collocation with low freq but high probability

14. Improved MWA Method • Add a penalization function y=f(x), x=freq(w1,w2) • When x is small, y approaches 0 (penalize) • When x is large, y approaches 1 (do not penalize) • y = e-b/x (b is tuned to 25) • New ranking score

15. Further Evaluation • Automatic evaluation • Greatly outperforms the best baseline • For top 1K, 20.6% vs. 11.7% • Exponential function plays a key role

16. Further Evaluation (2) • Human Evaluation • Top 1K collocations • For each collocation, tag "True" or "False" • 4 "False" cases • A: two semantically related words • (醫生, 護士) • B: a part of multi-word collocation(>= 3 words) • (自我, 機制) in (自我, 約束, 機制) • C: high frequency bigram • (他, 說), (這, 是), (很, 好) • D: two words co-occurring frequently • (北京, 月), (和, 為)

17. Further Evaluation (3) • True collocations are much more than baseline • False collocation • A: semantically related, not distinguishable by MWA • B: only two-word collocation is extracted. • Few collocations have >=3 words • C: frequent bigram, not distinguishable by MWA • D: much less than baseline

18. Further Evaluation (3) cont. • MWA are able to produce long-span collocations • 48 extracted collocations with span > 6 • 33 are tagged "True" • ("處於", "狀態"), ("由於", "因此") • 69% precision

19. Fertility vs. Precision • Manually label 100 sentences and observe fertility • 78% words collocate with 1 word • 17% words collocate with 2 words • 95% words have fertility <= 2 • Limit Φmax

20. Conclusion • Main contribution • Successfully adapt BWA to MWA • Propose a ranking method • Alignment probability + Exponential penalty function • Initial failure are well discussed • Future work • Improving Statistical Machine Translation with Monolingual Collocation, ACL 2010 • Improve alignment, phrase table