CS 479, section 1:Natural Language Processing

Lecture #36: Alignment and Metrics

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Thanks to Dan Klein of UC Berkeley and Uli Germann of ISI for many of the materials used in this lecture.

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Project Report: Early: Friday Due: Monday

Reading Report #14 Phrase-based MT paper Due: next Wednesday(online again)

Objectives Consider additional models for statistical word

alignment

See how our alignment models capture real phrases

Understand how to actually score a word-level alignment with a word alignment model

Discuss how to evaluate alignment quality

Quiz

Why do we use parallel corpora (bitexts)?

What is the hidden (unknown) variable in building translation models?

What was the main idea behind IBM Model 1?

Model 2?

Recall This Example

Des tremblements de terre ont à nouveau touché le Japon jeudi 4 novembre.

On Tuesday Nov. 4, earthquakes rocked Japan once again

What else is going on here that we haven’t tried to model?

Models Summary IBM Model 1: word alignment IBM Model 2: word alignment, with global position

(order) model HMM Model: word alignment, with local position IBM Model 3: adds model of fertility to model 2, deficient

IBM Model 4: adds relative ordering to model 3, deficient

IBM Model 5: fixes deficiency of Model 4

Context

Given a source language sentence, The search algorithm must propose possible

translations (target language sentences) along with corresponding alignments

Let’s pretend we’re in the midst of a search and scoring a single hypothesis

How do we use our models to compute such a score?

Example: How to score Spanish source sentence:

“Maria no daba una bofetada a la bruja verde” Here denotes “foreign” sentence.

During the search, we propose a possible English translation:“Mary did not slap the green witch”

We consider one possible alignment What is the score, according to model 5?

i.e., what is ?

Example: How to score

Mary not slap slap slap the green witch n(3|slap)

P(NULL)

t(la|the)

d(j|i)Maria no daba una bofetada a la bruja verde

Mary not slap slap slap NULL the green witch

Maria no daba una bofetada a la verde bruja

[Al-Onaizan and Knight, 1998]

Mary did not slap the green witchExamples from Local Models

Example: How to score

Mary not slap slap slap the green witch n(3|slap)

P(NULL)

t(la|the)

d(j|i)Maria no daba una bofetada a la bruja verde

Mary not slap slap slap NULL the green witch

Maria no daba una bofetada a la verde bruja

[Al-Onaizan and Knight, 1998]

Mary did not slap the green witch

Cascaded Training

Standard practice for training: Initialize one model with the previous (simpler)

model Proceed with EM

Typical order: 1, (2 | HMM), 3, 4, 5

Insight into these models

Examples: Translation and Fertility

Example: Idioms

Example: Morphology

Example: WSD Word sense disambiguation:

Word-based MT systems rarely have a WSD step Why not?

Choosing an Alignment

Choosing an Alignment

Efficiency of Choosing Best Alignment

Evaluating TMs

How do we measure TM quality?

Measure quality of the alignments produced Metric: AER

Alignment Error Rate

/ Actual

AER

Easy to measure

Problems? Hard to know what the gold alignments should be May not correlate with translation quality

like perplexity and speech reco. accuracy in LMs

AER Results[O

ch &

Ney

, 200

3] C

anad

ian

Hans

ards

dat

a

Next

Decoding

Complexity of Decoding

Evaluating Translation