Corpora and Translation

Parallel corpora

Statistical MT

(not to mention: Corpus of translated text, for translation studies)

2/38

Parallel corpora

• Corpora of texts and their translations• Basic idea that such parallel corpora implicitly

contain lots of information about translation equivalence

• Nowadays many such “bitexts” are available– bilingual countries have laws, parliamentary

proceedings, and other documents– large multinational organizations (UN, EU [Europarl

corpus], etc.)– multinational commercial organizations produce

multilingual texts

3/38

Bilingual concordance

Source: TransSearch,Laboratoire de Recherche Appliquée en Linguistique Informatique, Université de Montréal

http://www-rali.iro.umontreal.ca

4/38

Parallel corpora

• Usually not corpora in the strict sense (planned, annotated, etc.)

• Usefulness may depend on – the quality of translation– the closeness of translation– whether we have a text and its translation, or

a multilingually authored text– the language pair

• Parallel corpus needs to be aligned

5/38

Alignment

• Means annotating the bilingual corpus to show explicitly the correspondences– at sentence level– at word and phrase level

• Main difficulty for sentence alignment is that translations do not always keep sentence boundaries, or even sentence order

• In addition, translation may be “localized” and therefore not especially faithful

6/38

Sentence-level alignment

• If parallel corpus is quite a literal translation, this can be done using quite low-level information– sentence length– looking for anchors

• proper names, dates, figures• eg in a parliamentary debate, speakers’ names

7/38

Alignment tools

8/38

Corpus-based MT

• Translation memory (tool for translators)– database of previous translations– find close matching examples to current

translation unit– translator decides what to do with it

9/38

Note that translator has to know/decide what bits of the target sentence to change

10/38

Corpus-based MT

• Translation memory (tool for translators)– database of previous translations– find close matching examples to current

translation unit– translator decides what to do with it

• Example-based translation– similar idea, but computer program tries to

manipulate example(s)– may involve “learning” general rules from

multiple examples

11/38

Statistical MT

• Pioneered by IBM in early 1990s• Spurred on by better success in speech

recognition of statistical over linguistic rule-based approaches

• Idea that translation can be modelled as a statistical process

• Seems to work best in limited domain where given data is a good model of future translations

12/38

Translation as a probabilistic problem

• For a given SL sentence Si, there are number of “translations” T of varying probability

• Task is to find for Si the sentence Tj for which the probability P(Tj | Si) is the highest

13/38

Two models

• P(Tj | Si) is a function of two models:

– The probabilities of the individual words that make up Tj given the individual words in Si - the “translation model”

– The probability that the individual words that make up Tj are in the appropriate order – the “language model”

14/38

Expressed in mathematical terms:

Since S is a given, and constant, this can be

simplified as

)(

)|()()|(maxarg

SP

TSPTPSTP

)|()()|(maxarg TSPTPSTP

Translation modelLanguage model

15/38

So how do we translate?

• For a given input sentence Si we have to have a practical way to find the Tj that maximizes the formula

• We have to start somewhere, so we start with the translation model: which words look most likely to help us?

• In a systematic way we can keep trying different combinations together with the language model until we stop getting improvements

16/38

Input sentence

Translation model

Bag of possible words

Most probable

translation

Seek improvement by tryingother combinations

Language model

17/38

Where do the models come from?

• All the statistical parameters are pre-computed (“learned”), based on a parallel corpus

• Language model is probabilities of word sequences (n-grams)

• Translation model is derived from aligned parallel corpus

• This approach is attractive to some as an example of “machine learning”– The computer learns to translate (just) from seeing

previous examples of translation

18/38

The translation model

• Take sentence-aligned parallel corpus

• Extract entire vocabulary for both languages

• For every word-pair, calculate probability that they correspond – e.g. by comparing distributions

19/38

Problem: fertility

• “fertility”: not all word correspondences are 1:1 – Some words have multiple possible

translations, e.g. the {le, la, l’, les}– Some words have no translation, e.g. in il se

rase ‘he shaves’, se – Some words are translated by several words,

e.g. cheap peu cher– Not always obvious how to align

20/38

Problem: distortion

• Notice that corresponding words do not appear in the same order.

• The translation model includes probabilities for “distortion” – e.g. P(2|5): the P that ws in position 2 will

produce a wt in position 5– can be more complex: P(5|2,4,6): the P

that ws in position 2 will produce a wt in position 5 when S has 4 words and T has 6.

21/38

The language model

• Impractical to calculate probability of every word sequence:– Many will be very improbable …– Because they are ungrammatical– Or because they happen not to occur in the data

• Probabilities of sequences of n words (“n-grams”) more practical– Bigram model:

where P(wi|wi–1) f(wi–1, wi)/f(wi)

)|(),...,,( 121 iin wwPwwwP

22/38

Sparse data

• Relying on n-grams with a large n risks 0-probabilities

• Bigrams are less risky but sometimes not discriminatory enough– e.g. I hire men who is good pilots

• 3- or 4-grams allow a nice compromise, and if a 3-gram is previously unseen, we can give it a score based on the component bigrams (“smoothing”)

23/38

Put it all together and …?

• To build a statistical MT system we need:– Aligned bilingual corpus– “Training programs” which will extract from the

corpora all the statistical data for the models– A “decoder” which takes a given input, and

seeks the output that evaluates the magic argmax formula – based on a heuristic search algorithm

• Software for this purpose is freely available – http://www.statmt.org/moses/,

http://www.isi.edu/licensed-sw/pharaoh/

• Claim is that an MT system for a new language pair can be built in a matter of hours

24/38

SMT latest developments

• Nevertheless, quality is limited• SMT researchers quickly learned that this crude

approach can get them so far (quite far actually), but that to go the extra distance you need linguistic knowledge (eg morphology, “phrases”, consitutents)

• Latest developments aim to incorporate this• Big difference is that it too can be LEARNED

(automatically) from corpora• So SMT still contrasts with traditional RBMT

where rules are “hand coded” by linguists

25/38

Direct phrase alignment

(Wang & Waible 1998, Och et al., 1999, Marcu & Wong 2002)

• Enhance word translation model by adding joint probabilities, i.e. probabilities for phrases

• Phrase probabilities compensate for missing lexical probabilities

• Easy to integrate probabilities from different sources/methods, allows for mutual compensation

26/38

Word alignment induced model

Koehn et al. 2003; example stolen from Knight & Koehn http://www.iccs.inf.ed.ac.uk/~pkoehn/publications/tutorial2003.pdf

Maria did not slap the green witch

Maria no daba una botefada a la bruja verda

Start with all phrase pairs justified by the word alignment

27/38

Word alignment induced model

Koehn et al. 2003; example stolen from Knight & Koehn http://www.iccs.inf.ed.ac.uk/~pkoehn/publications/tutorial2003.pdf

(Maria, Maria), (no, did not)(daba una botefada, slap),(a la, the), (verde, green), (bruja, witch)

28/38

Word alignment induced model

Koehn et al. 2003; example stolen from Knight & Koehn http://www.iccs.inf.ed.ac.uk/~pkoehn/publications/tutorial2003.pdf

(Maria, Maria), (no, did not)(daba una botefada, slap),(a la, the), (verde, green) (bruja, witch), (Maria no, Maria did not), (no daba una botefada, did not slap),(daba una botefada a la, slap the), (bruja verde, green witch)

etc.

29/38

Word alignment induced model

Koehn et al. 2003; example stolen from Knight & Koehn http://www.iccs.inf.ed.ac.uk/~pkoehn/publications/tutorial2003.pdf

(Maria, Maria), (no, did not), (slap, daba una bofetada), (a la, the), (bruja, witch), (verde, green), (Maria no, Maria did not), (no daba una bofetada, did not slap),(daba una bofetada a la, slap the), (bruja verde, green witch),(Maria no daba una bofetada, Maria did not slap),(no daba una bofetada a la, did not slap the), (a la bruja verde, the green witch),(Maria no daba una bofetada a la, Maria did not slap the),(daba una bofetada a la bruja verde, slap the green witch),(no daba una bofetada a la bruja verde, did not slap the green witch),(Maria no daba una bofetada a la bruja verde, Maria did not slap the green witch)

30/38

Alignment templates

Och et al. 1999; further developed by Marcu and Wong 2002, Koehn and Knight 2003, Koehn et al. 2003)

• Problem of sparse data worse for phrases• So use word classes instead of words

– alignment templates instead of phrases– more reliable statistics for translation table– smaller translation table– more complex decoding

• Word classes are induced (by distributional statistics), so may not correspond to intuitive (linguistic) classes

• Takes context into account

31/38

Problems with phrase-based models

• Still do not handle very well ...– dependencies (especially long-distance)– distortion – discontinuities (e.g. bought = habe ... gekauft)

• More promising seems to be ...

32/38

Syntax-based SMT

• Better able to handle – Constituents– Function words– Grammatical context (e.g. case marking)

• Inversion Transduction Grammars

• Hierarchical transduction model

• Tree-to-string translation

• Tree-to-tree translation

33/38

Inversion transduction grammars

• Wu and colleagues (1997 onwards)

• Grammar generates two trees in parallel and mappings between them

• Rules can specify order changes

• Restriction to binary rules limits complexity

34/38

Inversion transduction grammars

35/38

Inversion transduction grammars

• Grammar is trained on word-aligned bilingual corpus: Note that all the rules are learned automatically

• Translation uses a decoder which effectively works like traditional RBMT:– Parser uses source side of transduction rules to build

a parse tree– Transduction rules are applied to transform the tree– The target text is generated by linearizing the tree

36/38

37/38

38/38

Other approaches

• Other approaches use more and more “linguistic” information

• In each case automatically learned, especially from treebanks

• Traditional (“rule-based”) MT used (hand-written) grammars and lexicons

• State-of-the-art MT is moving back in this direction, except that linguistic rules are machine learned