Historical Text Normalization with Delayed Rewards
What it is about
Sequence-to-sequence normalisers are usually trained with token-level maximum likelihood. The paper fine-tunes such a model with policy gradient, so that the training signal is the exact match of the whole normalisation, a delayed reward. Six historical datasets are used. Fine-tuning reduces the error by 20% for English. For German, Hungarian, Icelandic, Slovene and Swedish the reduction is 7% to 16%, still significant at p < 0.01. The gains are largest for long words, for unseen words and for inputs where the baseline tends to produce long outputs. Inputs that stay unchanged over time are handled less well for English.
Ideas
- I1.
en/src/morph.rs: a deterministic version of the reward. Give each rule a weight, and after each dev pass raise the weights of rules that give exact matches and lower the others. Keep all rule tables in BTreeMap order, and measure exact-match accuracy on tools/gutenberg dev lessons, run twice. - I2. Stratify before changing anything: report
en morph-evalerror rates by word length and by whether the word was seen in training, the two factors the paper finds. Use the report to choose which lessons to tune on. - I3. Keep the change log: for each rule-weight change, store the reward delta and the lesson it came from. This makes each gain traceable, which the paper's correlation analysis lacks.
For Mova
The chapter is 09. Early Modern and archaic forms. The code is en/src/morph.rs and en morph-eval. The paper's neural method is not for Mova; the exact-match reward is the idea to copy in a count-based form. Measure first the error rate of archaic split by word length.
Cite
@inproceedings{flachs2019historical,
title = {{Historical Text Normalization with Delayed Rewards}},
author = {Flachs, Simon and Bollmann, Marcel and Søgaard, Anders},
booktitle = {Proceedings of the 57th Annual Meeting of the Association for Computational Linguistics},
year = {2019},
doi = {10.18653/v1/p19-1157},
url = {https://doi.org/10.18653/v1/p19-1157}
}References
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