The Importance of Category Labels in Grammar Induction with Child-directed Utterances
Summary
This paper concerns the evaluation of grammars that a model induces from raw text without supervision. Such grammars are usually compared only by their bracketing structure (unlabeled F1). Phrasal category labels are ignored, even though they are an essential part of a grammar. The authors propose the Recall-Homogeneity (RH) metric. It multiplies the unlabeled constituent recall by the homogeneity of labels in the constituents that match the gold standard. Unlike RVM, RH does not penalize a model for categories that are finer-grained than the gold ones. The inducer is a Bayesian Dirichlet-multinomial PCFG model in Chomsky normal form with Gibbs sampling. The data are adult child-directed speech from the CHILDES corpus: English (Adam, Eve), Mandarin (Tong) and German (Leo). WSJ20Dev is also used for comparison. Under unlabeled evaluation, dense, almost purely right-branching grammars appear to be the best. RH, by contrast, confirms the linguistically expected advantage of sparse grammars and of a larger number of categories. Bounding center-embedding depth remains beneficial under labeled evaluation as well. Hence, conclusions about grammar induction that do not take labels into account may be wrong.
Key points
- Data: for each language, over 15,000 caregiver utterances from CHILDES. The English trees for Adam and Eve are hand-annotated. The gold trees for Tong and Leo were produced automatically by the Kitaev and Klein (2018) parser. Each model was run 10 times for 700 iterations.
- Parameter β (Dirichlet concentration): unlabeled F1 peaks at high β, whereas RH peaks at low β, which is consistent with the sparsity of human grammars. As an example, the authors cite the Penn Treebank: 73 nonterminal categories and only 17,020 attested rules out of over 28 million possible ones. On WSJ20Dev the trend is the same but weaker.
- Number of categories C: at β = 0.01, RH increases with C and peaks at C = 75. For comparison, Adam has 63 unique tags, while previous work used 30 categories or fewer.
- Depth bounding: on Adam, the best RH is achieved with depth 3, which is statistically significantly better than the unbounded model (p < 10⁻³). The result was confirmed on Eve, Tong and Leo: the difference in unlabeled F1 is significant at p < 0.001. In these experiments C = 30 due to hardware limitations.
Topics Psycholinguistics & language acquisition
Cite
Cite as Lifeng Jin and William Schuler. 2020. The Importance of Category Labels in Grammar Induction with Child-directed Utterances. Proceedings of the 16th International Conference on Parsing Technologies and the IWPT 2020 Shared Task on Parsing into Enhanced Universal Dependencies.
@inproceedings{jin2020importance,
title = {{The Importance of Category Labels in Grammar Induction with Child-directed Utterances}},
author = {Lifeng Jin and William Schuler},
booktitle = {Proceedings of the 16th International Conference on Parsing Technologies and the IWPT 2020 Shared Task on Parsing into Enhanced Universal Dependencies},
year = {2020},
doi = {10.18653/v1/2020.iwpt-1.15},
eprint = {2006.11646},
archivePrefix = {arXiv},
url = {https://doi.org/10.18653/v1/2020.iwpt-1.15}
}References
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