NLG Evaluation Metrics Beyond Correlation Analysis: An Empirical Metric Preference Checklist

Iftitahu Nimah, Meng Fang, Vlado Menkovski, Mykola Pechenizkiy


Abstract
In this study, we analyze automatic evaluation metrics for Natural Language Generation (NLG), specifically task-agnostic metrics and human-aligned metrics. Task-agnostic metrics, such as Perplexity, BLEU, BERTScore, are cost-effective and highly adaptable to diverse NLG tasks, yet they have a weak correlation with human. Human-aligned metrics (CTC, CtrlEval, UniEval) improves correlation level by incorporating desirable human-like qualities as training objective. However, their effectiveness at discerning system-level performance and quality of system outputs remain unclear. We present metric preference checklist as a framework to assess the effectiveness of automatic metrics in three NLG tasks: Text Summarization, Dialogue Response Generation, and Controlled Generation. Our proposed framework provides access: (i) for verifying whether automatic metrics are faithful to human preference, regardless of their correlation level to human; and (ii) for inspecting the strengths and limitations of NLG systems via pairwise evaluation. We show that automatic metrics provide a better guidance than human on discriminating system-level performance in Text Summarization and Controlled Generation tasks. We also show that multi-aspect human-aligned metric (UniEval) is not necessarily dominant over single-aspect human-aligned metrics (CTC, CtrlEval) and task-agnostic metrics (BLEU, BERTScore), particularly in Controlled Generation tasks.
Anthology ID:
2023.acl-long.69
Volume:
Proceedings of the 61st Annual Meeting of the Association for Computational Linguistics (Volume 1: Long Papers)
Month:
July
Year:
2023
Address:
Toronto, Canada
Editors:
Anna Rogers, Jordan Boyd-Graber, Naoaki Okazaki
Venue:
ACL
SIG:
Publisher:
Association for Computational Linguistics
Note:
Pages:
1240–1266
Language:
URL:
https://aclanthology.org/2023.acl-long.69
DOI:
10.18653/v1/2023.acl-long.69
Bibkey:
Cite (ACL):
Iftitahu Nimah, Meng Fang, Vlado Menkovski, and Mykola Pechenizkiy. 2023. NLG Evaluation Metrics Beyond Correlation Analysis: An Empirical Metric Preference Checklist. In Proceedings of the 61st Annual Meeting of the Association for Computational Linguistics (Volume 1: Long Papers), pages 1240–1266, Toronto, Canada. Association for Computational Linguistics.
Cite (Informal):
NLG Evaluation Metrics Beyond Correlation Analysis: An Empirical Metric Preference Checklist (Nimah et al., ACL 2023)
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PDF:
https://aclanthology.org/2023.acl-long.69.pdf
Video:
 https://aclanthology.org/2023.acl-long.69.mp4