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Unifying Human and Statistical Evaluation for Natural Language Generation

Paper · 2019 · Tatsunori B. Hashimoto, Hugh Zhang and Percy Liang

Summary

Evaluating text generators is hard because the usual tools each miss something. Human ratings reward fluent output but cannot tell when a model just copies training data or always produces the same answer. Perplexity rewards coverage but barely punishes occasional bad samples. Hashimoto, Zhang and Liang define one target quantity: twice the error of the best classifier that tries to tell reference text from model output. A perfect model gets 1, and a model with no overlap with the reference gets 0. They prove this optimal classifier needs only two values per sentence: its probability under the model and its probability under the reference. Since the reference probability cannot be computed, they replace it with crowdsourced "typicality" ratings. HUSE is the leave-one-out error of a k-nearest-neighbour classifier over the length-normalised model log-probability and the human score. Experiments on summarization and chit-chat dialogue show two things:

  • HUSE exposes diversity problems that pure human evaluation misses.
  • Quality-boosting tricks like annealing actually lower HUSE.

Key points

  • The target metric L* equals 1 minus the total variation distance between the model and reference distributions. HUSE is an upper bound on it that is never looser than human-only evaluation: L(φ_hj) ≥ L(φ_huse) ≥ L*.
  • The features are log p_model(y|x) divided by sentence length, plus HJ(x, y). HJ is the average 1–5 typicality rating from 20 Amazon Mechanical Turk workers.
  • On the Reddit corpus, HJ correlated strongly with a sentence's log-frequency (r = 0.92), which supports using it as a proxy for log p_ref.
  • The tasks studied were language modeling, chit-chat dialogue, story generation and summarization.
  • Code, data and experiments are released on CodaLab.

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

 Fields, topics and concepts

Cite

Cite as Tatsunori B. Hashimoto, Hugh Zhang and Percy Liang. 2019. Unifying Human and Statistical Evaluation for Natural Language Generation. arXiv:1904.02792.

The paper itself, not this page. BibTeX from the library's record of the work. DOI 10.48550/arxiv.1904.02792 · arXiv 1904.02792

@misc{hashimoto2019unifying,
  title = {{Unifying Human and Statistical Evaluation for Natural Language Generation}},
  author = {Tatsunori B. Hashimoto, Hugh Zhang and Percy Liang},
  year = {2019},
  doi = {10.48550/arxiv.1904.02792},
  eprint = {1904.02792},
  archivePrefix = {arXiv},
  url = {https://doi.org/10.48550/arxiv.1904.02792}
}

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