Lexically Grounded Subword Segmentation

Jindřich Libovický, Jindřich Helcl


Abstract
We present three innovations in tokenization and subword segmentation. First, we propose to use unsupervised morphological analysis with Morfessor as pre-tokenization. Second, we present an algebraic method for obtaining subword embeddings grounded in a word embedding space. Based on that, we design a novel subword segmentation algorithm that uses the embeddings, ensuring that the procedure considers lexical meaning. Third, we introduce an efficient segmentation algorithm based on a subword bigram model that can be initialized with the lexically aware segmentation method to avoid using Morfessor and large embedding tables at inference time. We evaluate the proposed approaches using two intrinsic metrics and measure their performance on two downstream tasks: part-of-speech tagging and machine translation. Our experiments show significant improvements in the morphological plausibility of the segmentation when evaluated using segmentation precision on morpheme boundaries and improved Rényi efficiency in 8 languages. Although the proposed tokenization methods do not have a large impact on automatic translation quality, we observe consistent performance gains in the arguably more morphological task of part-of-speech tagging.
Anthology ID:
2024.emnlp-main.421
Volume:
Proceedings of the 2024 Conference on Empirical Methods in Natural Language Processing
Month:
November
Year:
2024
Address:
Miami, Florida, USA
Editors:
Yaser Al-Onaizan, Mohit Bansal, Yun-Nung Chen
Venue:
EMNLP
SIG:
Publisher:
Association for Computational Linguistics
Note:
Pages:
7403–7420
Language:
URL:
https://aclanthology.org/2024.emnlp-main.421
DOI:
Bibkey:
Cite (ACL):
Jindřich Libovický and Jindřich Helcl. 2024. Lexically Grounded Subword Segmentation. In Proceedings of the 2024 Conference on Empirical Methods in Natural Language Processing, pages 7403–7420, Miami, Florida, USA. Association for Computational Linguistics.
Cite (Informal):
Lexically Grounded Subword Segmentation (Libovický & Helcl, EMNLP 2024)
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PDF:
https://aclanthology.org/2024.emnlp-main.421.pdf