CSA4050: Advanced Topics in NLP

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CSA4050Advanced Topics in NLP

• Example Based MT

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Example Based MT

• Man does translation, first, by properly
  decomposing an input sentence into certain
  fragmental phrases, then by translating these
  phrases into other language phrases, and finally
  by properly composing these fragmental
  translations into one long sentence.
• Nagao, 1984

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Example

• Problem translateHe buys a book on
  international politics
• Database
• He buys a notebookKare wa noto o
  kauhe topic notebook object buys
• I read a book on international politicsWatachi
  wa kokusai seiji nitsuite kakareta hon o yomu
• Answerkare wa kokusai seiji nitsuite kakareta
  hon o kau.

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Translation Process

• Input he buys a book on international politics
• Indentify/translate fragments
• he buyskare wa .. kau
• a book on international politicskokusai seiji
  nitsuite kakareta hon
• Combine translated fragments.kare wa kokusai
  seiji nitsuite kakareta hon kau

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Three Step Process

• Match identify relevant source language examples
  in database.
• Align find corresponding fragments in target
  language.
• Recombine target language fragments to form
  sentences.

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Matching

• Nature of matching process depends on database
  organisation.
• In the simplest case it is simply a bilingual
  corpus that has been aligned at sentence level.
• Effectively this means that the database is a
  collection of sentence pairs.
• Identification of relevant pairs is carried out
  by matching sentences.

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Matching

• Different methods of sentence/sentence matching
• Character Based
• Word Based
• Structure Based
• Partial

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Character Based

• Problem semantic edit distance versus character
  edit distance
• Paper tray A holds up to 400 sheets
• Paper tray B holds up to 400 sheets
• The large paper tray
• The small paper tray

d1
d5
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Word Based

• Allows matching between different words on the
  basis of a similarity metric (e.g based on
  semantic features).
• This allows inexact matches and also solves the
  problem of which stored translation to choose.
• In the following case which involve different
  translations for the word eat in Japanese.
• StoredA man eats vegetables.Acid eats
  metal.Example to translateHe eats potatoes

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Matching Fragments

He buys a book on international
politics
he buys a notebook he buys a horse he buys a car
for his mum he buys a politician John buys
toothpaste
he reads a book on international politics books
on international poltics are exciting John sold a
a book on antiques
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Bibliography

• Example Based MTH Somers Review
  ArticleMachine Translation 14.2