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# ACL2014 Reading: [Zhang+] "Kneser-Ney Smoothing on Expected Count" and [Pickhardt+] "A Generalized Language Model as the Comination of Skipped n-grams and Modified Kneser-Ney Smoothing"

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### ACL2014 Reading: [Zhang+] "Kneser-Ney Smoothing on Expected Count" and [Pickhardt+] "A Generalized Language Model as the Comination of Skipped n-grams and Modified Kneser-Ney Smoothing"

1. 1. [Zhang+ ACL2014] Kneser-Ney Smoothing on Expected Count [Pickhardt+ ACL2014] A Generalized Language Model as the Comination of Skipped n-grams and Modified Kneser-Ney Smoothing 2014/7/12 ACL Reading @ PFI Nakatani Shuyo, Cybozu Labs Inc.
2. 2. Kneser-Ney Smoothing [Kneser+ 1995] • Discounting & Interpolation 𝑃 𝑤𝑖 𝑤𝑖−𝑛+1 𝑖−1 = max 𝑐 𝑤𝑖−𝑛+1 𝑖 − 𝐷, 0 𝑐 𝑤𝑖−𝑛+1 𝑖−1 + 𝐷 𝑐 𝑤𝑖−𝑛+1 𝑖−1 𝑁1+ 𝑤𝑖−𝑛+1 𝑖−1 ∙ 𝑃 𝑤𝑖 𝑤𝑖−𝑛+2 𝑖−1 • where 𝑤 𝑚 𝑛 = 𝑤 𝑚 ⋯ 𝑤 𝑛, 𝑁1+ 𝑤 𝑚 𝑛 ⋅ = 𝑤𝑖|𝑐 𝑤 𝑚 𝑛 𝑤𝑖 > 0 Number of Discounting
3. 3. Modified KN-Smoothing [Chen+ 1999] 𝑃 𝑤𝑖 𝑤𝑖−𝑛+1 𝑖−1 = 𝑐 𝑤𝑖−𝑛+1 𝑖 − 𝐷 𝑤𝑖−𝑛+1 𝑖 𝑐 𝑤𝑖−𝑛+1 𝑖−1 + 𝛾 𝑤𝑖−𝑛+1 𝑖−1 𝑃 𝑤𝑖 𝑤𝑖−𝑛+2 𝑖−1 • where 𝐷 𝑐 = 0 if 𝑐 = 0, 𝐷1 if 𝑐 = 1, 𝐷2 if 𝑐 = 2, _ 𝐷3+ if 𝑐 ≥ 3 𝛾 𝑤𝑖−𝑛+1 𝑖−1 = [amount of discounting] 𝑐 𝑤𝑖−𝑛+1 𝑖−1 Weighted Discounting (D_n are estimated by leave-1-out CV)
4. 4. [Zhang+ ACL2014] Kneser-Ney Smoothing on Expected Count • When each sentence has fractional weight – Domain adaptation – EM-algorithm on word alignment • Propose KN-smoothing using expected fractional counts I’m interested in it!
5. 5. Model • 𝒖 means 𝑤𝑖−𝑛+1 𝑖−1 , and 𝒖′ means 𝑤𝑖−𝑛+2 𝑖−1 • A sequence 𝒖𝑤 occurs 𝑘 times and each occurring has probability 𝑝𝑖 (𝑖 = 1, ⋯ , 𝑘) as weight, • then count 𝑐(𝒖𝑤) is distributed according to Poisson Binomial Distribution. • 𝑝 𝑐 𝑢𝑤 = 𝑟 = 𝑠 𝑘, 𝑟 , where 𝑠 𝑘, 𝑟 = 𝑠 𝑘 − 1, 𝑟 1 − 𝑝 𝑘 + 𝑠 𝑘 − 1, 𝑟 − 1 𝑝 𝑘 if 0 ≤ 𝑟 ≤ 𝑘 1 if 𝑘 = 𝑟 = 0 0 otherwise
6. 6. MLE on this model • Expectations – 𝔼 𝑐 𝒖𝑤 = 𝑟 ⋅ 𝑝 𝑐 𝒖𝑤 = 𝑟𝑟 – 𝔼 𝑁𝑟 𝒖 ⋅ = 𝑝 𝑐 𝒖𝑤 = 𝑟𝑤 – 𝔼 𝑁𝑟+ 𝒖 ⋅ = 𝑝 𝑐 𝒖𝑤 ≥ 𝑟𝑤 • Maximize (expected) likelihood – 𝔼 𝐿 = 𝔼 𝑐 𝒖𝑤 log 𝑝 𝑤 𝒖𝒖𝑤 = 𝔼 𝑐 𝒖𝑤 log 𝑝 𝑤 𝒖𝒖𝑤 – obtain 𝑝MLE 𝑤 𝒖 = 𝔼 𝑐 𝒖𝑤 𝔼 𝑐 𝒖⋅
7. 7. Expected Kneser-Ney • 𝑐 𝒖𝑤 = max 0, 𝑐 𝒖𝑤 − 𝐷 + 𝑁1+ 𝒖 ⋅ 𝐷𝑝′(𝑤|𝒖′ ) • So, 𝔼 𝑐 𝒖𝑤 = 𝔼 𝑐 𝒖𝑤 − 𝑝 𝑐 𝒖𝑤 > 0 𝐷 + 𝔼 𝑁1+ 𝒖 ⋅ 𝐷𝑝′(𝑤|𝒖′ ) – where 𝑝′ 𝑤 𝒖′ = 𝔼 𝑁1+ ⋅𝒖′ 𝑤 𝔼 𝑁1+ ⋅𝒖′⋅ • then 𝑝 𝑤 𝒖 = 𝔼 𝑐 𝒖𝑤 𝔼 𝑐 𝒖⋅
8. 8. Language model adaptation • Our corpus consists on – large general-domain data and – small specific domain data • Sentence 𝒘 ‘s weight: – 𝑝 𝒘 is in − domain = 1 1+exp −𝐻 𝒘 – where 𝐻 𝒘 = log 𝑝in 𝒘 −log 𝑝out 𝒘 𝒘 , – 𝑝in:lang. model of in-domain, 𝑝out: out’s one
9. 9. • Figure 1: On the language model adaptation task, expected KN outperforms all other methods across all sizes of selected subsets. Integral KN is applied to unweighted instances, while fractional WB, fractional KN and expected KN are applied to weighted instances. (via [Zhang+ ACL2014]) from general-domain data in-domain data - training: 54k - testing: 3k 192 162 156 148 Why isn't there Modified KN as a baseline?
10. 10. [Pickhardt+ ACL2014] A Generalized Language Model as the Comination of Skipped n-grams and Modified Kneser-Ney Smoothing • Higher-order n-grams are very sparse – Especially remarkable on small data(e.g. domain specific data!) • Improve performance for small data by skipped n-grams and Modified KN- smoothing – Perplexity reduces 25.7% for very small training data of only 736KB text
11. 11. “Generalized Language Models” • 𝜕3 𝑤1 𝑤2 𝑤3 𝑤4 = 𝑤1 𝑤2_𝑤4 – “_” means a word placeholder 𝑃GLM 𝑤𝑖 𝑤𝑖−𝑛+1 𝑖−1 = 𝑐 𝑤𝑖−𝑛+1 𝑖 − 𝐷 𝑐 𝑤𝑖−𝑛+1 𝑖 𝑐 𝑤𝑖−𝑛+1 𝑖−1 +𝛾high 𝑤𝑖−𝑛+1 𝑖−1 1 𝑛 − 1 𝑃GLM 𝑛−1 𝑗=1 𝑤𝑖 𝜕𝑗 𝑤𝑖−𝑛+1 𝑖−1 𝑃GLM 𝑤𝑖 𝜕𝑗 𝑤𝑖−𝑛+1 𝑖−1 = 𝑁1+ 𝜕𝑗 𝑤𝑖−𝑛 𝑖 − 𝐷 𝑐 𝜕𝑗 𝑤𝑖−𝑛+1 𝑖 𝑁1+ 𝜕𝑗 𝑤𝑖−𝑛+1 𝑖−1 ∗ +𝛾mid 𝜕𝑗 𝑤𝑖−𝑛+1 𝑖−1 1 𝑛 − 2 𝑃GLM 𝑤𝑖 𝜕𝑗 𝜕 𝑘 𝑤𝑖−𝑛+1 𝑖−1 𝑛−1 𝑘=1,𝑘≠𝑗
12. 12. • The bold arrows correspond to interpolation of models in traditional modified Kneser-Ney smoothing. The lighter arrows illustrate the additional interpolations introduced by our generalized language models. (via [Pickhardt+ ACL2014])
13. 13. • shrunk training data sets for the English Wikipedia small domain specific data
14. 14. Space Complexity model size = 9.5GB # of entries = 427M model size = 15GB # of entries = 742M
15. 15. References • [Zhang+ ACL2014] Kneser-Ney Smoothing on Expected Count • [Pickhardt+ ACL2014] A Generalized Language Model as the Comination of Skipped n-grams and Modified Kneser-Ney Smoothing • [Kneser+ 1995] Improved backing-off for m- gram language modeling • [Chen+ 1999] An Empirical Study of Smoothing Techniques for Language Modeling