Evaluating historical text normalization systems: How well do they generalize?

We highlight several issues in the evaluation of historical text normalization systems that make it hard to tell how well these systems would actually work in practicei.e., for new datasets or languages; in comparison to more na\"ive systems; or as a preprocessing step for downstream NLP tools. We illustrate these issues and exemplify our proposed evaluation practices by comparing two neural models against a na\"ive baseline system. We show that the neural models generalize well to unseen words in tests on five languages; nevertheless, they provide no clear benefit over the na\"ive baseline for downstream POS tagging of an English historical collection. We conclude that future work should include more rigorous evaluation, including both intrinsic and extrinsic measures where possible.