Comparable Corpora Kashyap Popat( ) Rahul Sharnagat(11305R013)

Outline  Motivation  Introduction: Comparable Corpora  Types of corpora  Methods to extract information from comparable corpora  Bilingual dictionary  Parallel sentences  Conclusion

Motivation  Corpus: the most basic requirement in statistical NLP  Large amount of bilingual text on web  Bilingual Dictionary generation  One to one correspondence between words  Parallel Corpus generation  One to one correspondence between sentences  Very rare resource (Hindi – Chinese)

Comparable corpora [7]  “A comparable corpus is one which selects similar texts in more than one language or variety. There is as yet no agreement on the nature of the similarity, because there are very few examples of comparable corpora.”  Characteristics of Comparable corpora  No parallel sentences  No parallel paragraphs  Fewer overlapping terms and words Definition by EAGLES

Spectrum of Corpora Unrelated corpora Comparable corporaParallel corporaTranscription - sentence by sentence aligned

A comparable corpora

Application of comparable corpora  Generating bilingual lexical entries (dictionary)  Creating parallel corpora

Generating bilingual lexical entries

Basic postulates [1]  Words with productive context in one language translate to word with productive context in second language e.g., table  मेज़  Words with rigid context translate into words with rigid context e.g., Haemoglobin  रक्ताणु  Correlation: between co-occurrence pattern in different languages Compiling Bilingual Lexicon Entries From a Non-Parallel English-Chinese Corpus, Fung, 1995

Co-occurrence patterns [4]  If a term A co-occurs with another term B in some text T then its translation A' also co-occurs with B‘(translation of B) in some other text T' Automatic Identification of Word Translations from Unrelated English and German Corpora. R. Rapp, 1999 T T’ A B A’ B’

Co-occurrence Histogram [2]  For the word ‘Debenture’ Words Count Finding terminology translations from non-parallel corpora. Fung, 1997

Basic Approach [3]  Calculate the co-occurrence matrix for all the words in source language L 1 and target language L 2  Word order of the L 1 matrix is permuted until the resulting pattern is most similar to that of the L 2 matrix Identifying word translations in nonparallel, Rapp, R.,1995

English co-occurrence matrix  L 1 matrix Book1 Garden2 Plant3 School4 Sky5 Teacher6

Hindi co-occurrence matrix  L 2 matrix आकाश 1 पाठशाला 2 शिक्षक 3 बगीचा 4 किताब 5 पौधा 6

Hindi co-occurrence matrix  L 2 matrix: after permutations किताब 5 बगीचा 4 पौधा 6 पाठशाला 2 आकाश 1 शिक्षक 3

Result  Comparing the order of the words in L 1 matrix and permuted L 2 matrix L 1 indexWord in L 1 Word in L 2 L 2 index 1Book किताब 5 2Garden बगीचा 4 3Plant पौधा 6 4School पाठशाला 2 5Sky आकाश 1 6Teacher शिक्षक 3

Problems  Permuting co-occurrence matrix is expensive  Size of the vector # of unique terms in the language

A New Method [2]  Dictionary entries are used as seed words to generate correlation matrices  Algorithm:  A bilingual list of known translation pairs (seed words) is given  Step-1: For every word ‘e’ in L 1, find its correlation vector (M 1 ) with every word of L 1 in the seed words  Step-2: For every words ‘c’ in L 2, find its correlation vector (M 2 ) with every word of L 2 in the seed words  Step-3: Compute correlation(M 1, M 2 ); if it is high, ‘e’ and ‘c‘ are considered as a translation pair Finding terminology translations from non-parallel corpora. Fung, 1997

Co-occurrence EnglishHindi Garden बगीचा …… Plant पौधा …… …… Sky आकाश …… …… …… Seed word List Flower फूल

Crux of the Algorithm  Two main steps:  Formation of co-occurrence matrix  Measuring the similarity between vectors  Different possible methods to calculate above two steps  Advantage: vector size reduces to # of unique words in the seed list

Improvements  Window Size for co-occurrence calculation [2]  Should it be same for all the words ?  Co-occurrence Counts  Similarity Measure Finding terminology translations from non-parallel corpora. Fung, 1997

Co-occurrence count  Mutual Information (Church & Hanks, 1989)  Conditional Probability (Rapp, 1996)  Chi-Square Test (Dunning, 1993)  Log-likelihood Ratio (Dunning, 1993)  TF-IDF (Fung et al 1998) Automatic Identification of Word Translations from Unrelated English and German Corpora, R. Rapp.,1999

Mutual Information [2] (1/2) k 11 = # of segments where both, w s and w t occur k 12 = # of segments where only w s occur k 21 = # of segments where only w t occur k 22 = # of segments where neither words occur  Segments: sentences, paragraphs, or string groups delimited by anchor paints Finding terminology translations from non-parallel corpora. Fung, 1997

Mutual Information [2] (2/2)  Weighted mutual information

Similarity Measures(1/2)  Cosine similarity (Fung and McKeown,1997)  Jaccard similarity (Grefenstette,1994)  Dice similarity (Rapp, 1999)  L1 norm / City block distance (Jones & Furnas, 1987)  L2 norm / Euclidean distance (Fung, 1997) Automatic Identification of Word Translations from Unrelated English and German Corpora, R. Rapp.,1999

Similarity Measures(2/2)  L1 norm / City block distance  L2 norm / Euclidian distance  Cosine Similarity  Jaccard Similarity

Problems with the approach [5]  Coverage: only few corpus words are covered by the dictionary  Synonymy / Polysemy: several entries have the same meaning (synonymy), or an entry has several meanings (polysemy)  Similarities w.r.t. synonyms should not be independent  Improvements in the form of Geometric approaches  Projects the co-occurrence vectors of source and target word on a dictionary entries  Measures the similarity between the projected vectors A geometric view on bilingual lexicon extraction from comparable corpora. Gaussier, et al., 2004

Results PaperApproachMethodCorpusAccuracy Fung et al. 1996Word list based Best candidateEnglish/Japanese29% Word list based Top 20 candidate output English/Japanese50.9% Gaussier et al GeometricAvg. PrecisionEnglish/French44% R. Rapp et al Word list based 100 Test words English /French72%

Generating parallel corpora

Generating Parallel Corpora  Involves aligning the sentences in the comparable corpora to form a parallel corpora  Ways to do:  Dictionary matching  Statistical methods

Ways to do alignment  Dictionary matching  If the words in given two sentences are translation of each other, it is most likely that the sentences are translation of each other  Process is very slow  Accuracy is high but cannot be applied to large corpus  Statistical methods  To predict the alignment, these methods make use of distribution of length of sentence in corpus either in terms of words (Brown, 1996) or characters (Gale and Church, 1991)  Makes no use of any lexical resources  Fast and accurate

Length based statistical approach  Preprocessing  Segment the text into tokens  Combine the token into groups (nothing but sentences)  Find anchor point  Find points in corpus, where we are sure that start and end points in one language of the corpus aligns to start and end points in other language of the corpus  Finding these points require analysis of corpus

Example  Brown et al., already had anchors in their corpus  Used UK parliament proceedings ‘Hansards‘ as a parallel corpus  Each proceeding start with a comment, time of proceeding, who was giving the speech etc.  This information provides the anchor points. Sample text from Aligning Sentences in parallel corpora, P. Brown, Jeniffer Lei and Robert Mercer,1996

Aligning anchor points  Anchor points are not always perfect  Some may be missing  Some may be garbled  To find the alignment between these anchors, Dynamic programming technique is used  We find an alignment of the major anchors in the two corpora with the least total cost

Beads  Upper level view can be that corpus is sequence of sentence lengths occasionally separated by paragraph markers  Each of these groupings is called a bead  Bead is a type of sentence grouping Sample text from Aligning Sentences in parallel corpora, P. Brown, Jeniffer Lei and Robert Mercer,1996

Beads Bead typeContent eOnly one English sentence fOnly one French sentence efOne English and one French sentence eefTwo English and one French sentence effOne English and two French sentence ¶eOne English paragraph ¶fOne French paragraph ¶e¶fOne English and one French paragraph Example of beads

Problem Formulation  Sentences between the anchored points get generated by two random processes 1. Producing a sequence of beads 2. Choosing the length of the sentence(s) in each bead  Bead generation can be modeled using a two state Markov model  One sentence can align to zero, one or two sentence in the other side  Allows any of the eight beads as shown in previous table  Assumptions,

Modeling length of sentence  Model probability of length of sentence given its bead  Assumptions are made:  e-beads and f-beads: Probability of l e or l f is same as probability of l e or l f in the whole corpus  ef-bead:  English sentence: length l e with probability Pr(l e )  French sentence: that log ratio of French to English sentence length is normal distributed with mean µ and variance Where r = log(l f | l e )

Contd..  eef-bead:  English sentence: drawn from Pr(l e )  French sentence: r is distributed according to same normal distribution  eff-bead: same uniform distribution holds with r as  English sentence: drawn from Pr(l e )  French sentence: r is distributed according to same normal distribution  Given the sum of lengths of French sentences, probability for particular pair l f1 and l f2 is proportional to

Parameter Estimation  Using EM Algorithm, estimate the parameters of the Markov model  Following results were obtained Sample text from Aligning Sentences in parallel corpora, P. Brown, Jeniffer Lei and Robert Mercer,1996

Results  In a random sample of 1000 sentences, only 6 were not translation of each other  Brown et al. have also studied the effect of anchors points  According to them,  with paragraph marker but no anchor points, 2.0% error rate is expected  with anchor points but no paragraph marker, 2.3% error rate is expected  with neither anchor point nor paragraph marker, 3.2% error is rate expected

Conclusion  Comparable corpora can be used to generate bilingual dictionary and parallel corpora  Generating bilingual dictionary  Polysemy and sense disambiguation still remains a major challenge  Generating parallel corpora  Given the aligned points, aligner is likely to give good results  The experiments were very specific to corpora, hard to generalize the accuracy  The sentences of length which has a highest chance to get aligned but with completely wrong translation might confuse the aligner

References 1. Fung, P. (1995). Compiling bilingual lexicon entries from a non-parallel English-Chinese corpus. Proceedings of the 3rd Annual Workshop on Very Large Corpora, Boston, Massachusetts, Fung, P.; McKeown, K. (1997). Finding terminology translations from non-parallel corpora. Proceedings of the 5th Annual Workshop on Very Large Corpora, Hong Kong, R. Rapp (1995). Identifying word translations in nonparallel texts. In: Proceedings of the 33rd Meeting of the Association for Computational Linguistics. Cambridge, Massachusetts, R. Rapp. (1999). Automatic Identification of Word Translations from Unrelated English and German Corpora. Proceedings of the ACL-99. pp. 1–17. College Park, USA.

References 5. Gaussier, Eric, Jean-Michel Renders, Irina Matveeva, Cyril Goutte, and Herve Dejean. (2004). A geometric view on bilingual lexicon extraction from comparable corpora. In Proceedings of the 42nd Annual Meeting of the Association for Computational Linguistics, pages 527–534, Barcelona, Spain. 6. Peter F. Brown, Jennifer C. Lai, and Robert L. Mercer (1991). Aligning sentences in parallel corpora. In Proceedings of the 29th annual meeting on Association for Computational Linguistics (ACL '91)