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LLM-based NLG Evaluation: Current Status and Challenges

Paper · 2025 · Mingqi Gao, Xinyu Hu, Xunjian Yin et al.

Summary

Judging natural language generation is difficult because a good output has no single correct form and must satisfy several quality criteria at once. Overlap metrics like BLEU, and even model-based scores like BARTScore, agree poorly with human judgments and are easy to fool. This Peking University survey reviews roughly 100 studies, collected from Google Scholar up to June 2024 and including many arXiv preprints, that use LLMs with more than one billion parameters as evaluators of tasks such as translation, summarization, dialogue and data-to-text. The authors group the work into four families: scores computed from LLM embeddings or token probabilities, prompting existing LLMs, fine-tuning LLMs on labelled evaluation data, and evaluation shared between humans and LLMs. For each family they weigh robustness, efficiency and bias. A side-by-side comparison on SummEval shows that the strongest LLM-based judges clearly outperform classic metrics, with cost rising alongside quality. The survey ends by calling for unified benchmarks, research beyond English, and new forms of human–LLM collaboration. These gaps matter directly for building evaluators for Ukrainian and other under-resourced languages.

Key points

  • Taxonomy and example methods:
    • LLM-derived metrics: GPTScore sums token log-probabilities of the output given an instruction and context. FFLM and DELTA-SCORE measure how probabilities shift under changed conditions. Es et al. compute embedding similarity with text-embedding-ada-002.
    • Prompting: G-Eval.
    • Fine-tuning: INSTRUCTSCORE, Prometheus 2, Themis, TIGERScore, CompassJudger-1.
    • Human–LLM collaboration: InteractEval.
  • SummEval correlations (overall score):
    • BERTScore 0.231 and BARTScore 0.329.
    • GPTScore (Qwen-2.5, 72B) 0.436.
    • G-Eval (GPT-4) 0.523 and Themis (8B) 0.553.
    • InteractEval (GPT-4) reaches about 0.71–0.73, the best result.
  • Cost ranking, from cheapest to most expensive:
    • LLM-derived metrics and prompting open-source models cost about the same.
    • Fine-tuning costs roughly as much as prompting proprietary APIs.
    • Human–LLM collaboration is the most expensive.
  • Weaknesses of LLM-derived metrics:
    • Stress tests (He et al. 2023) expose blind spots.
    • Closed models do not expose logits, so these metrics cannot be applied to them.
    • Model-based metrics carry more social bias across six sensitive attributes (Sun et al. 2022).
  • Gaps for future work:
    • Almost all research targets English, and authors urge caution for non-Latin scripts. Fine-tuning on low-resource languages looks promising (Hada et al. 2024).
    • Existing human-judgment sets are small, cover a single task, and contain outputs from outdated systems such as Pointer Network.

Our summary of the paper, in our own words — not its text. The work belongs to its authors; read it at the links above.

 Institutions

Cite

Cite as Mingqi Gao, Xinyu Hu, Xunjian Yin, Jie Ruan, Xiao Pu and Xiaojun Wan. 2025. LLM-based NLG Evaluation: Current Status and Challenges. Computational Linguistics.

The paper itself, not this page. BibTeX from the library's record of the work. DOI 10.1162/coli_a_00561

@article{gao2025llm,
  title = {{LLM-based NLG Evaluation: Current Status and Challenges}},
  author = {Mingqi Gao, Xinyu Hu, Xunjian Yin, Jie Ruan, Xiao Pu and Xiaojun Wan},
  journal = {Computational Linguistics},
  year = {2025},
  doi = {10.1162/coli\_a\_00561},
  url = {https://doi.org/10.1162/coli_a_00561}
}

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