Know When to Fuse: Investigating Non-English Hybrid Retrieval in the Legal Domain

Antoine Louis, Gijs van Dijck, Gerasimos Spanakis


Abstract
Hybrid search has emerged as an effective strategy to offset the limitations of different matching paradigms, especially in out-of-domain contexts where notable improvements in retrieval quality have been observed. However, existing research predominantly focuses on a limited set of retrieval methods, evaluated in pairs on domain-general datasets exclusively in English. In this work, we study the efficacy of hybrid search across a variety of prominent retrieval models within the unexplored field of law in the French language, assessing both zero-shot and in-domain scenarios. Our findings reveal that in a zero-shot context, fusing different domain-general models consistently enhances performance compared to using a standalone model, regardless of the fusion method. Surprisingly, when models are trained in-domain, we find that fusion generally diminishes performance relative to using the best single system, unless fusing scores with carefully tuned weights. These novel insights, among others, expand the applicability of prior findings across a new field and language, and contribute to a deeper understanding of hybrid search in non-English specialized domains.
Anthology ID:
2025.coling-main.290
Volume:
Proceedings of the 31st International Conference on Computational Linguistics
Month:
January
Year:
2025
Address:
Abu Dhabi, UAE
Editors:
Owen Rambow, Leo Wanner, Marianna Apidianaki, Hend Al-Khalifa, Barbara Di Eugenio, Steven Schockaert
Venue:
COLING
SIG:
Publisher:
Association for Computational Linguistics
Note:
Pages:
4293–4312
Language:
URL:
https://aclanthology.org/2025.coling-main.290/
DOI:
Bibkey:
Cite (ACL):
Antoine Louis, Gijs van Dijck, and Gerasimos Spanakis. 2025. Know When to Fuse: Investigating Non-English Hybrid Retrieval in the Legal Domain. In Proceedings of the 31st International Conference on Computational Linguistics, pages 4293–4312, Abu Dhabi, UAE. Association for Computational Linguistics.
Cite (Informal):
Know When to Fuse: Investigating Non-English Hybrid Retrieval in the Legal Domain (Louis et al., COLING 2025)
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PDF:
https://aclanthology.org/2025.coling-main.290.pdf