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How Much Does Tokenization Affect Neural Machine Translation

TLDR
The conclusion is reached that the tokenization significantly affects the final translation quality and that the best tokenizer differs for different language pairs.
Abstract
Tokenization or segmentation is a wide concept that covers simple processes such as separating punctuation from words, or more sophisticated processes such as applying morphological knowledge. Neural Machine Translation (NMT) requires a limited-size vocabulary for computational cost and enough examples to estimate word embeddings. Separating punctuation and splitting tokens into words or subwords has proven to be helpful to reduce vocabulary and increase the number of examples of each word, improving the translation quality. Tokenization is more challenging when dealing with languages with no separator between words. In order to assess the impact of the tokenization in the quality of the final translation on NMT, we experimented on five tokenizers over ten language pairs. We reached the conclusion that the tokenization significantly affects the final translation quality and that the best tokenizer differs for different language pairs.

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Citations
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Posted Content

Neural Machine Translation: A Review

TL;DR: This work traces back the origins of modern NMT architectures to word and sentence embeddings and earlier examples of the encoder-decoder network family and concludes with a survey of recent trends in the field.
Proceedings ArticleDOI

How Good is Your Tokenizer? On the Monolingual Performance of Multilingual Language Models

TL;DR: The authors provide a systematic and comprehensive empirical comparison of pretrained multilingual language models versus their monolingual counterparts with regard to their monolinguistic task performance, and find that while the pretraining data size is an important factor in the downstream performance, a designated mon-olingual tokenizer plays an equally important role in downstream performance.
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Byte Pair Encoding is Suboptimal for Language Model Pretraining

TL;DR: Differences between BPE and unigram LM tokenization are analyzed, finding that the latter method recovers subword units that align more closely with morphology and avoids problems stemming from BPE’s greedy construction procedure.
Proceedings ArticleDOI

Investigating the Effectiveness of BPE: The Power of Shorter Sequences.

TL;DR: The experiments show that - given a fixed vocabulary size budget - the fewer tokens an algorithm needs to cover the test set, the better the translation (as measured by BLEU).
Posted Content

How Good is Your Tokenizer? On the Monolingual Performance of Multilingual Language Models.

TL;DR: This paper provided a systematic and comprehensive empirical comparison of pretrained multilingual language models versus their monolingual counterparts with regard to their monolinguistic task performance, and found that while the pretraining data size is an important factor in the downstream performance of the multilingual model, a designated mon-olingual tokenizer plays an equally important role in downstream performance.
References
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TL;DR: It is conjecture that the use of a fixed-length vector is a bottleneck in improving the performance of this basic encoder-decoder architecture, and it is proposed to extend this by allowing a model to automatically (soft-)search for parts of a source sentence that are relevant to predicting a target word, without having to form these parts as a hard segment explicitly.