a monolingual tree-based translation model for sentence simplification

124.08.2010| Computer Science Department | UKP Lab - Prof. Dr. Iryna Gurevych | Zhemin Zhu |

Zhemin Zhu, UKP, TU Darmstadt, GermanyDelphine Bernhard, LIMSI-CNRS, FranceIryna Gurevych, UKP, TU Darmstadt, Germany

A Monolingual Tree-based Translation Model for Sentence Simplification

Presenter: Zhemin Zhu

COLING2010 – Beijing, China

224.08.2010 | Computer Science Department | UKP Lab - Prof. Dr. Iryna Gurevych | Zhemin Zhu |

An Example of Sentence Simplification

This month was first called Sextilis in Latin, because it was the sixth month in the old Roman calendar. The Roman calendar began in March about 735 BC with Romulus.

-- Simple Wikipedia

This month was originally named Sextilis in Latin, because it was the sixth month in the original [ten-month] Roman calendar under Romulus in 753 BC, when March was the first month of the year.

-- Wikipedia

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Sentence Simplification Targeted at Humans

Reading and Speech Assistance

People with Comprehension Disabilities [Carroll et al., 1999; Inui et al., 2003]

Low-literacy people[Watanabe et al., 2009]

Non-native Speakers [Siddharthan, 2002]

Children 24.08.2010 | Computer Science Department | UKP Lab - Prof. Dr. Iryna Gurevych | Zhemin Zhu |

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Sentence Simplification Targeted at NLP Applications

Parsing and Translation [Chandrasekar et al., 1996]

Summarization[Knight and Marcu, 2000]

Sentence Fusion[Filippova and Strube, 2008b]

Semantic Role Labeling[Vickrey and Koller, 2008]


Question Generation[Heilman and Smith, 2009]

Relation Extraction[Miwa et al., COLING2010]

Information Extraction [Jonnalagadda and Gonzalez, 2009]

Robot Command[Young KY and Liu SH, 2002]

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What Makes a Sentence Difficult?


1. Difficult Vocabulary→ Vocabulary (Word/Phrase) Substitution

2. Complex Syntax Length → Splitting, Dropping Order → Reordering, such as passive and active

Simplification operations: Splitting, Dropping, Reordering and Substitution

This month was originally named Sextilis in Latin, because it was the sixth month in the original ten-month Roman calendar under Romulus in 753 BC, when March was the first month of the year.

-- Wikipedia

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Simplification Operation: Sentence Splitting


August is the eighth month of the year in the Gregorian Calendar and one of seven Gregorian months with a length of 31 days.

-- Wikipedia

August is the eighth month of the year.It has 31 days.

-- Simple Wikipedia

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Simplification Operation: Dropping


April is the fourth month of the year [in the Gregorian Calendar, and one of four months] with [a length of] 30 days.

-- Wikipedia

April is the fourth month of the year with 30 days.

-- Simple Wikipedia

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Simplification Operation: Reordering


Mr. Anthony, who runs an employment agency, decries program trading, but he isn't sure it should be strictly regulated.

-- [Siddharthan, 2006]

Mr. Anthony decries program trading. Mr. Anthony runs an employment agency.But he isn't sure it should be strictly regulated.

-- [Siddharthan, 2006]

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Simplification Operation: Substitution


The traditional etymology is from the Latin aperire, "to open," in allusion to its being the season when trees and flowers begin to "open," which is supported by comparison with the modern Greek use of ἁνοιξις (opening) for spring.

-- Wikipedia

The name April comes from that Latin word aperire which means "to open".

-- Simple Wikipedia

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Motivation

Most of the existing methods only cover one simplification operation: [Siddharthan, 2006] and [Petersen and Ostendorf , 2007]: Splitting Sentence Compression: Dropping [Carroll et al. ,1999]: Word Substitution

In most cases, different simplification operations happen simultaneously.

It is necessary to model different simplification operations integrally.


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Our Contributions

The first statistical model: TSM (Tree-based Simplification Model) Integrally covering splitting, dropping, reordering and word/phrase substitution Based on the great successes of parsing and translation techniques.

An Efficient Training Method for TSM Speeding up by monolingual word mapping

PWKP : Parallel Complex-Simple Dataset Obtained from Wikipedia and Simple Wikipedia


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Tree-base Simplification Model: TSM

Splitting

Dropping

Reordering

Substitution


Parse Trees of Complex Sentences

SimpleSentences

Probabilistic Model: EM Training

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Parallel Complex-Simple Dataset: PWKP

Paired articles from the Wikipedia and Simple Wikipedia

1. Article Pairing: following the “language links”

2. Plain Text Extraction: JWPL [Zesch et al., 2008]

3. Pre-processing: sentence boundary detection and tokenization with the Stanford Parser package [Klein and Manning, 2003], lemmatization with the TreeTagger [Schmid,1994]

4. Monolingual Sentence Alignment: sentence-level TF*IDF [Nelken and Shieber, 2006]


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Parallel Complex-Simple Dataset: PWKP


Similarity Precision Recall

TF*IDF 91.3% 55.4%

Word Overlap 50.5% 55.1%

MED 13.9% 54.7%

Table 1: Monolingual Sentence Alignment

Sentence Length Token Length #Pairs

Simple 20.87 4.89108016Complex 25.01 5.06

Table 2: Statistics for the PWKP dataset

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TSM: Splitting


Example Complex Sentence:

August was the sixth month in the ancient Roman calendar which started in 735BC.

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TSM: Splitting

Question 1: Where to split the sentence? Step 1: Segmentation

Question 2: How to make the split sentences complete and grammatical? Step 2: Completion


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TSM: Splitting

Step 1: Segmentation


Word Constituent Length Probability

which SBAR 1 0.0016

which SBAR 2 0.0835

Table 3: Segmentation Feature Table (SFT)

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TSM: Splitting

Step 1: Segmentation


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TSM: Splitting

Step 2: Completion

Should the “which” be dropped?


Word Constituent isDropped Probability

which WHNP true 1.0

which WHNP false Prob.min

Table 4: Border Drop Feature Table (BDFT)

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TSM: Splitting

Step 2: Completion

Which parts should be copied? Where to put these parts in the new sentences?


Dependency Constituent isCopied Position Probability

gov_nsubj VBD true left 0.9000

gov_nsubj VBD true right 0.0994

gov_nsubj VBD false left + right 0.0006

Table 5: Copy Feature Table (CFT)

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TSM: Splitting


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TSM: Dropping & Reordering


Constituent Children Drop Probability

NP DT JJ NNP NN 1101 7.66E-4

NP DT JJ NNP NN 0001 1.26E-7

Table 6: Dropping Feature Table (DFT)

Constituent Children Reorder Probability

NP DT JJ NN 012 0.8303

NP DT JJ NN 210 0.0039

Table 7: Reordering Feature Table (RFT)

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TSM: Dropping & Reordering


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TSM: Word/Phrase Substitution


Original (word/phrase)

Substitution(word/phrase)

Probability

ancient ancient 0.963

ancient old 0.0183

old ancient 0.005

ancient than transportation 1.83E-102

Table 8: Substitution Feature Table (SubFT)

Word substitution: terminal nodes

Phrase Substitution: non-terminal nodes

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TSM: Word/Phrase Substitution


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Speeding up

We filter out the unpromising candidates at the early stages. This is done using monolingual word mapping.


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Experiments

Testing dataset:100 complex sentences

131 parallel simple sentences from PWKP

Baseline systems:1. Moses: state-of-the-art phrase-based SMT

2. Compression (Filippova and Strube, 2008a)

3. Compression + Substitution Substitution: Wordnet + Frequency in Simple Wikipedia Articles

4. Compression + Substitution + Splitting Splitting: split at conjunctions and relatives.


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Experiments: Basic Statistics


Tok. Len. Sent . Len. #Sent.

Complex Sentences 4.95 27.81 100

Simple Sentences 4.76 17.86 131

1. Moses 4.81 26.08 100

2. Compression 4.98 18.02 103

3. Compression+Substitution 4.90 18.11 103

4. Compression+Substitution+splitting 4.98 10.20 182

5. TSM 4.76 13.57 180

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Experiments: Translation Assessment


BLEU NIST #Same

Complex Sentences 0.50 6.89 100

Simple Sentences 1.00 10.98 3

1. Moses 0.55 7.47 25

2. Compression 0.28 5.37 1

3. Compression+Substitution 0.19 4.51 0

4. Compression+Substitution+splitting 0.18 4.42 0

5. TSM 0.38 6.21 2

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Experiments: Readability Assessment


Flesch Lix (Grade)

OOV% PPL

Complex Sentences 49.1 53.0 (10) 52.9 384

Simple Sentences 60.4 (PE) 44.1 (8) 50.7 179

1. Moses 54.8 48.1 (9) 52.0 363

2. Compression 56.2 45.9 (8) 51.7 481

3. Compression+Substitution 59.1 45.1 (8) 49.5 616

4. Compression+Substitution+splitting 65.5 (PE) 38.3 (6) 53.4 581

5. TSM 67.4 (PE) 36.7 (5) 50.8 353

PE: Plain English Grade: School Year

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Conclusions

1. Moses is not good at simplification tasks.

2. BLEU and NIST are not a good evaluation metrics for sentence simplification systems.

3. TSM can achieve the best overall readability scores.

4. We contributed the PWKP dataset:

http://www.ukp.tu-darmstadt.de/software-data/data/quality-assessment/




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Future Work

More sophisticated features and rules to improve TSM

Extend TSM’s expressiveness to model more complex transformations: synchronous syntax is a promising direction

Evaluation methods for simplification systems: Readability Assessment


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Acknowledgements


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Thanks for your interests!

Comments & Questions!


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Backup: Training

EM algorithm:


Training (dataset){Initialize all probability tables using the uniform distribution;for (several iterations){

reset all cnt = 0;for (each sentence pair < c; s > in dataset){

tt = buildTrainingTree(< c; s >);calcInsideProb(tt);calcOutsideProb(tt);update cnt for each conditioning feature in eachnode of tt: cnt = cnt + node:insideP rob node:outsideP rob=root:insideP rob;

}updateProbability();

}}

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Backup: Training


a monolingual tree-based translation model for sentence simplification

Documents

franceiryna gurevych

sentence difficult

simple wikipedia

zhemin zhucoling2010

sentence splitting24

sentence fusionfilippova

gregorian calendar

gregorian months