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A Statistical Model for Domain-Independent Text Segmentation

Paper · 2001 · Masao Utiyama and Hitoshi Isahara

Summary

Utiyama and Isahara address text segmentation, the task of splitting a document into topically coherent parts. They target settings with no labelled data, such as summarizing documents or speeches. Earlier unsupervised methods relied on lexical cohesion heuristics like cosine similarity or word repetition. This paper instead treats segmentation as a probabilistic search problem. Each segment is modelled as its own word distribution, estimated only from the text being segmented and smoothed with Laplace's law. A prior over segmentations comes from a minimum-description-length argument: the cost of transmitting the segment lengths. Together these give a cost for every candidate segment. Finding the best segmentation then reduces to a minimum-cost path through a graph of inter-word positions, solved with dynamic programming. The method also picks the number of segments automatically. On Choi's artificial benchmark, the system matched or beat Choi's (2000) state-of-the-art segmenter, with statistically significant gains. This matters because it offers a principled, training-free segmenter that works in any domain and can later absorb supervised features.

Key points

  • The model assumes topics are statistically independent and words are independent given their topic. Word probabilities within a segment use Laplace's law, so no external training corpus is needed.
  • The prior is uninformative, based on description length. Each segment length costs log n bits for a text of n words, giving k·log n bits for k segments. This term counterbalances the likelihood, which on its own would over-segment.
  • Segments correspond to edges in a graph of word-gap positions. The best segmentation is a minimum-cost path found by DP, with boundaries restricted here to sentence ends. A fixed number of segments can be enforced by limiting the number of edges in the path.
  • The number of segments grows only slowly with text length. A ~27-sentence editorial yields roughly 4–6 segments, while a text of over 1000 sentences yields about 10–20.
  • On 700 samples from Choi's artificial corpus, with the segment count given beforehand, the overall error was about 9% versus 11% for Choi's system (one-sided t-test, p ≈ 6.8E-9). The proposed system was better on every data subset reported. The implementation was publicly released by CRL.

Interesting ideas

  • I1. Segmentation as shortest path over word gaps. If segment scores are additive, any segmentation-style problem becomes a DP over a position graph. This could apply to sentence chunking, dialogue turn grouping, or splitting long Ukrainian documents for retrieval.
  • I2. MDL as a free prior. Charging bits for encoding each segment length provides a parameter-free penalty against too many segments. This trick is reusable wherever a model needs an automatic stopping criterion without tuning.
  • I3. Estimating from the input itself. Fitting smoothed per-segment word distributions on the document alone makes the method language- and domain-agnostic. It is attractive for low-resource languages or niche domains like trade documents, where annotated data is scarce.
  • I4. Pluggable segment-level priors. Because whole segments are scored directly, features of a segment can be added as extra cost terms. The authors show one example: a normal-distribution prior on segment length. This is easier than in HMM topic-tagging approaches, which only model transitions.
  • I5. By-product word densities. The per-segment word probabilities could help locate passages where a term is discussed in depth. This might support glossary extraction or term-definition mining.

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

Cite

Cite as Masao Utiyama and Hitoshi Isahara. 2001. A Statistical Model for Domain-Independent Text Segmentation. Proceedings of the 39th Annual Meeting of the Association for Computational Linguistics.

The paper itself, not this page. BibTeX from the library's record of the work. DOI 10.3115/1073012.1073076

@inproceedings{utiyama2001statistical,
  title = {{A Statistical Model for Domain-Independent Text Segmentation}},
  author = {Masao Utiyama and Hitoshi Isahara},
  booktitle = {Proceedings of the 39th Annual Meeting of the Association for Computational Linguistics},
  year = {2001},
  doi = {10.3115/1073012.1073076},
  url = {https://doi.org/10.3115/1073012.1073076}
}

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