Confidence Estimation for Machine Translation

J. Blatz et.al,Coling 04

SSLI MTRG 11/17/2004

Takahiro Shinozaki

Abstract

Detailed study of CE for machine translation Various machine learning methods CE for sentences and for words Different definitions of correctness

Experiments NIST 2003 Chinese-to-English MT

evaluation

1 Introduction

CE can improve usability of NLP based systems

CE techniques is not well studied in Machine translation

Investigate sentence and word level CE

2 Background Strong vs. weak CE

CE Score

Threshold Binary output

CE Score

Threshold Binary outputCorrectness probabilities

Strong CE: require probability

Weak CE: require only binary classification

NOT necessary probability

2 Background Has CE layer or not

No distinct CE layer

Has distinct CE Layer

NLP systemx y

NLP systemx y

CE module ScoreConfidence

ScoreConfidence

Naïve Bayes, NN, SVM etc…

Require a training corpus

Powerful and modular

3 Experimental Setting

Src

Hyp

Input sentences

Translation system

ISI Alignment Template MT system

N-best

Validation

Train

Test

Reference sentences

C

Correct or Not

3.1 Corpora Chinese-to-English

Evaluation sets from NIST MT competitions Multi reference corpus from LDC

3.2 CE Techniques Data : A collection of pairs (x,c)

X: feature vector, c: correctness Weak CE

X score X MLP score (Regressing MT evaluation score)

Strong CE X naïve Bayes P(c=1|x) X MLP P(c=1|x)

3.2 Naïve Bayes (NB) Assume features are statistically

independent

Apply absolute discounting

D

dd cxPcPcxPcPxcP

1

|||C

x1 x2 xD

3.2 Multi Layer Perceptron Non-linear mapping of input features

Linear transformation layers Non-linear transfer functions

Parameter estimation Weak CE (Regression)

• Target: MT evaluation score• Minimizing a squared error loss

Strong CE (Classification)• Target: Binary correct/incorrect class• Minimizing negative log likelihood

3.3 Metrics for Evaluation

Strong CE metric:Evaluates probability distribution Normalized cross entropy (NCE)

Weak CE metrics:Evaluates discriminability Classification error rate (CER) Receiver operating characteristic (ROC)

3.3 Normalized Cross Entropy Cross Entropy (negative log-likelihood)

i

ii xcPNLL |log

Normalized Cross Entropy (NCE)

n

n

n

n

n

n

n

nNLL

NLL

NLLNLLNCE

b

b

b

1100 loglog

Estimated probability from

CE module

Empirical probability

obtained from test set

3.3 Classification Error Rate

CER: Ratio of samples with wrong binary (Correct/Incorrect) prediction

Threshold optimization Sentence-level experiments: test set Word-level experiments: validation set

Baseline n

nnCERb

10 ,min

3.3 Receiver operating characteristic

Correct Incorrect

Correct a b

Incorrect c d

Prediction

Fact

ba

aratioacceptCorrect

dc

dratiorejectCorrect

ca

aPrecision

ba

aRecall

Cf.

0,0 1

1

random

ROC curve

IROC

Bette

r

Correct-reject-ratio

Corr

ect

-acc

ept-

rati

o

4 Sentence Level Experiments

MT evaluation measures WERg: normalized word error rate NIST: sentence-level NIST score

“Correctness” definition Thresholding WERg Thresholding NIST

Threshold value 5% “correct” examples 30% “correct” examples

4.1 Features

Total of 91 sentence level features Base-Model-Intrinsic

• Output from 12 functions for Maximum entropy based base-system• Pruning statistics

N-best List• Rank, score ratio to the best, etc…

Source Sentence• Length, ngram frequency statistics, etc…

Target Sentence• LM scores, parenthesis matching, etc…

Source/Target Correspondence• IBM model1 probabilities, semantic similarity, etc…

4.2 MLP Experiments MLPs are trained on all features for the fo

ur problem settings

Classification models are better than regression model Performance is better than baseline

Strong CE(Classification)

Weak CE(Regression) N/A

BASE CER3.21

32.5

5.65

32.5

N:NIST

W:WERg

Table 2

4.3 Feature Comparison

Compare contributions of features Individual feature Group of features

All: All features Base: base model scores BD: base-model dependent BI: base model independent S: apply to source sentence T: apply to target sentence ST: apply to source and target sentence

4.3 Feature Comparison (results)

Base All BD > BI T>ST>S CE Layer > No CE Layer

ALLBaseBDBISTST

Table 3

Figure 1

Exp. Condition: NIST 30%

5 Word Level Experiments Definition of word correctness

A word is correct if: Pos: occurs exactly at the same position

as reference WER: aligned to reference PER: occurs in the reference

Select a “best” transcript from multiple references

Ratio of “correct” words Pos(15%) < WER(43%) < PER(64%)

5.1 Features Total of 17 features

SMT model based features (2)• Identity of alignment template, whether or not translated by a rule

IBM model 1 (1)• Averaged word translation probability

Word posterior and Related measures (3x3)

Target language based features (3+2)• Semantic features by WordNet• Syntax check, number of occurrences in the sentence

Relative freq. Rank weighted freq. Word Posterior prob.

Any

Source

Target

WPP-any

WPP-source

WPP-target

5.2 Performance of Single Features

Experimental setting Naïve Bayes classifier PER based correctness

Table 4

WPP-any give the best results

WPP-any>model1>WPP-source

Top3>any of the single features

No gain for ALL

5.3 Comparison of Different models

Naïve Bayes, MLPs with different number of hidden units All features, PER based correctness

Naïve Bayes MLP0

Naïve Bayes < MLP5

MLP5 NLP10 NLP20

Figure 2

5.4 Comparison of Word Error Measures

Experimental settings MLP20 All features

PER is the easiest to lean

Table 5

6 Conclusion Separate CE layer is useful Features derived from base model are bet

ter than external ones N-best based features are valuable Target based features are more valuable t

han those not MLPs with hidden units are better than naï

ve Bayes