A Comparison of Unsuperviesed Bilingual Term Extraction Methods Using Phrase Tables

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Published on March 12, 2014

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A Comparison of Unsupervised Bilingual Term Extraction Methods Using Phrase-Tables Masamichi Ideue† Kazuhide Yamamoto Masao Utiyama Eiichiro Sumita ‡ Nagaoka University of Technology, Japan † National Institutre of Information and Communications Technology † ‡ ‡

Background • Automatic bilingual term extraction • Helpful for human translators • Applicable to other NLP tasks Develop unsupervised methods for extracting bilingual terms from a phrase-table, and compare them. Goal 1

Related works • Using parallel corpus • Using existing bilingual dictionary Tonoike et al. (2006) translated the number of word in each source language term using the bilingual dictionary and combined these translations to form term candidates. 2 Itagaki et al. (2007) proposed a supervised method for extracting bilingual terms from the phrase-table built from a parallel corpus. We usually do not have annotated data for training supervised methods nor bilingual dictionaries specific to the documents under translation.

3

Statistical measures 4 Three statistical scores are used to eliminate the wrong pairs, respectively. : Significance of the candidates based on Fisher’s exact test. ScoreF : Strength of the alignment between words of the candidates. ScoreL : Termhood of the candidate based on C-value. ScoreC

Bilingual term counting and combination of measures • Combination of scores 5 , 2 , 2 , , Score ( ) (Score ( )) (Score ( )) (Score ( )) 3 FLC J E F J E L J E C J E T R T R T R T = + + • Two methods for counting the number of occurrences of term T Method 1 : Counting without regarding where T occurs Method 2 : Counting T only when it occurs alone, i.e., we do not count the number of occurrences of term T when it occurs as a substring of a longer term.

Experiments 100 bilingual term candidates that were randomly selected from the top 1,000 candidates were manually evaluated for each score. A : correct A' : correct depending on contexts B : partly correct C : incorrect Evaluation criterion 6 • 22,543 bilingual term candidates were extracted from the Phrase-table. • Training corpus : Japanese-English parallel corpus, consisting of about 60,000 pairs, related to apparel products.

Translation accuracy A A' B C F1 43 25 24 8 L1 77 5 18 0 C 78 6 14 2 F2 71 18 8 3 L2 79 4 17 0 FLC 87 2 11 0 , , , and can filter the extracted noisy bilingual term. 7 2ScoreF ScoreFLC ScoreC2ScoreL

Characteristics of extracted bilingual terms occurrences words F2 Many Few L2 Many Many C Few Many FLC Few Many • Each measure extracts different bilingual term candidates. • The characteristic of indicated a tendency similar to . From this, the ’s residual noise was filtered by and . 8 ScoreFLC 2ScoreF 2ScoreL ScoreC ScoreC

Conclusion We compared three statistical measures for extracting bilingual terms from the phrase- table built from a parallel corpus. Each method differs in the number of words and the occurrences of bilingual terms. The combination of these measures ranks valid bilingual terms highly. 9

Fisher's exact test : Significance of the candidates • Fisher’s exact test has been used by Johnson et al. (2007) to select valid phrase pairs from the phrase-table for statistical machine translation. We use the statistic of Fisher’s exact test as Score_F to measure the validity of each bilingual term candidate. If Score_F of a bilingual term candidate is high, the candidate has the validity. ScoreF

Score_F C(J,E) C(J)-C(J,E) C(J) C(E)-C(J,E) N-C(J)-C(E)+C(J,E) N-C(J) C(E) N-C(E) N N : All parallel sentences C(J) : Japanese sentences containing J C(E): English sentences containing E C(J,E) : The number of parallel sentences containing J and E • P_h (C(J, E)) is the probability of observing the contingency table under the null hypothesis of J and E being independent of each other.

Log-likelihood Ratio • Tonoike et al. (2007) said alignments of a component of the term is useful for automatic bilingual term extraction. : Strength of the alignmentScoreL Using the word alignments of each candidate term to measure the validity of the candidates.

Alignment information We used the alignment information produced by Moses (Koehn et al., 2007). Alignments in the parallel sentences Alignments in ,J ET

C-Value If the term candidates of both language are highly ranked in C-value ranking, the bilingual term candidate has validity. : Termhood of the candidate color denim pants (C-Value = 6.34) color denim (2.0) denim pants (60.33) ScoreC The C-value (Frantzi et al., 1996) has been used to measure the stability of nested multi- word term candidates.

Bilingual term counting and combination of measures Our experiments show that the counting method is better than normal counting and characteristics of each measure are different. Therefore, we combine them. • Combination of measures • Two methods for counting the number of occurrences of term T Method 1 : Counting without regarding where T occurs Method 2 : Counting T only when it occurs alone, i.e., we do not count the number of occurrences of term T when it occurs as a substring of a longer term. 2 , 2 , , , (Score ( )) (Score ( )) (Score ( )) Score ( ) 3 F J E L J E C J E FLC J E R T R T R T T + + =

Examples of the extracted bilingual term daiya diamond daun jaketto down jacket kitake nagame long length wanpi- su one- piece kata osi embosse d leather ga-ze sozai gauze material siagari finish kisetu kan seasonal look kobana gara floral pattern pointo accent iro zukai coloring pasu ke-su card case B uesuto bubun (waist part) waist konbou sozai (blend material) blend iro oti (faced color) faded look C sodeguti (cuff) hem siruetto bodi- (body silhouette) item features A' F2 L2 C A

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