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Parallel Iterative Edit Models for
Local Sequence Transduction
A b h i j e e t A w a s t h i , S u n i t a S a r a w a g i , R a s n a G o ya l ,
S a b ya s a c h i G h o s h , V i h a r i P i r a t l a
I n t r o d u c e r : H i r o k i H o m m a , i n K o m a c h i ’s l a b .
J a n u a r y 2 7 t h , 2 0 2 0

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Abstract
概 要
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• They present a Parallel Iterative Edit (PIE) model
for the problem of local sequence transduction
arising in tasks like Grammatical error correction (GEC).
• not encoder-decoder (ED) model
• It achieves accuracy competitive with the ED model for four
reasons:
1. predicting edits instead of tokens
2. labeling sequences instead of generating sequences
3. iteratively refining predictions to capture dependencies
4. factorizing logits over edits and their token argument to harness pretrained language
models like BERT
• They have experimented with tasks that span GEC, OCR fixes, and spelling corrections.
• The PIE model has proven to be an accurate and very fast alternative to local sequence
transformation.
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Abstract

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Abstract

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Abstract

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Method
提 案 手 法
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Overview of the PIE model
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Method
(C) copy 𝑥𝑖
(A) append a q-gram 𝑤 ∈ Σ 𝑎
(D) delete
(R) replace 𝑥𝑖 with q-gram 𝑤 ∈ Σ 𝑎
(T𝑘) word-inflection

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The Seq2Edits Function
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Method

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• The Parallel Edit Prediction Model
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Method

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Experiments
実 験
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Grammatical error correction
settings 1
• train: Lang-8 + NUCLE + FCE corpus
• 1.2 million sentence pairs in English
• validation: CoNLL-13 test set
1. initializing: BERT-LARGE model
2. synthetic training: One-Billion-word corpus (for 2 epochs)
3. fine-tuning: real GEC training corpus (for 2 epochs)
• batch size: 64, learning rate: 2e-5
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Experiments

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Artificial Error Generation
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Experiments

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Grammatical error correction
settings 2 “edit space”
• copy
• delete
• 1000 appends
• 1000 replaces
• 29 transformations and their inverse
• evaluation
• MaxMatch (M2) scores (F0.5) : CONLL-2014-test
• GLEU+ scores: JEFLEG corpus
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Experiments
punctuations, articles, pronouns,
prepositions, conjunctions, verbs
add suffix s, d, es, ing, ed
replace suffix s to ing, d to s etc.

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Suffix transformations
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Experiments

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Grammatical error correction
result
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Experiments
ED
ensemble
decoding

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Grammatical error correction
Running Time Comparison
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Experiments
PIE models to be
considerably faster than
ED models

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Grammatical error correction
Impact of Iterative Refinement
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Experiments
test set: 1312 sentences
The average number of refinement rounds per
example was 2.7. In contrast, a sequential
model on this dataset would require 23.2 steps.

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Grammatical error correction
Ablation study on the PIE Architecture
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Experiments

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More Sequence Transduction Tasks
• Spell Correction
• Correcting OCR errors
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Experiments

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Conclusion
結 論
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• They presented a parallel iterative edit (PIE)
model for local sequence transduction with a
focus on the GEC task.
• Compared to the popular encoder-decoder models that
perform sequential decoding, parallel decoding in the PIE
model yields a factor of 5 to 15 reduction in decoding time.
• The PIE model employs a number of ideas to match the accuracy of
sequential models in spite of parallel decoding:
• it predicts in-place edits using a carefully designed edit space, iteratively refines its own
predictions, and effectively reuses state-of-the-art pre-trained bi-directional language models.
• In the future, they plan to apply the PIE model to more ambitious transduction tasks
like translation.
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Conclusion