Ties Matter: Meta-Evaluating Modern Metrics with Pairwise Accuracy and Tie Calibration
Paper · 2023 · Daniel Deutsch, George Foster and Markus Freitag
Summary
Segment-level evaluation of machine translation metrics usually relies on some form of Kendall's τ. Each variant handles tied scores differently, and that choice is becoming important. Stronger MT systems produce many error-free outputs, and MQM annotations use integer error counts. Both produce frequent ties in human scores. Newer metrics such as GEMBA (LLM prompting) and MaTESe also output only a few discrete values. Using WMT'22 MQM ratings for en→de, zh→en and en→ru, the authors show three things:
- No existing τ variant handles simple tie situations well.
- WMT's τ variants never reward a metric for correctly predicting a tie.
- Some variants can be gamed.
They propose two fixes. The first is acc_eq, a pairwise accuracy that counts matching ties as correct. The second is tie calibration, which searches for a threshold ε below which two metric scores count as tied. This lets metrics with continuous outputs compete fairly with discrete ones. The combination, acc*_eq, ranks metrics without systematic bias toward or against ties. The approach should carry over to evaluating other generative-AI metrics.
Key points
- Data and ties: The study uses WMT'22 MQM data with 13–15 systems per language pair. Under group-by-item grouping, 42–53% of human-score pairs are ties, and most of these are pairs of error-free translations.
- Ties by metric type: Regression metrics (Metric-X, COMET-22) tie on roughly 0.1–1.3% of pairs. MaTESe and the GEMBA metrics tie on about 40–81% of pairs.
- Gaming τ₁₀: Randomly breaking ties raises τ₁₀ for MaTESe from −0.459 to about 0.15, with a similar gain for GEMBA-GPT-3.5. COMET-22 improves by only about 0.005. Under acc*_eq, MaTESe and the GEMBA metrics move up from the bottom of the ranking to 4th, 6th and 15th.
- Transfer of ε: With WMT'21 used as held-out data, ε transfers well for en-de, with about a 2% relative change in accuracy. It fails for zh-en, where the share of ties differs between years (23% vs 41%).
- Where ties land and code: Calibrated ties mostly fall on high-scoring, near-perfect translations. Code is released in google-research/mt-metrics-eval.
📚 Shelves 4 Computer Science · 9,086 works Artificial Intelligence · 8,889 works Arts and Humanities · 9,296 works Language and Linguistics · 9,206 works
Cite
Cite as Daniel Deutsch, George Foster and Markus Freitag. 2023. Ties Matter: Meta-Evaluating Modern Metrics with Pairwise Accuracy and Tie Calibration. Proceedings of the 2023 Conference on Empirical Methods in Natural Language Processing.
@inproceedings{deutsch2023ties,
title = {{Ties Matter: Meta-Evaluating Modern Metrics with Pairwise Accuracy and Tie Calibration}},
author = {Daniel Deutsch, George Foster and Markus Freitag},
booktitle = {Proceedings of the 2023 Conference on Empirical Methods in Natural Language Processing},
year = {2023},
doi = {10.18653/v1/2023.emnlp-main.798},
url = {https://doi.org/10.18653/v1/2023.emnlp-main.798}
}References
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