Dynamic Topic Models
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Summary
Classic topic models such as LDA treat a collection as an unordered bag of documents. That ignores the fact that archives like journals, news and email change their content over time. Blei and Lafferty propose dynamic topic models (DTM), which split a corpus into time slices (e.g., years) and let each of K topics drift gradually from one slice to the next. They do not use Dirichlet priors, because these do not chain well over time. Instead they place Gaussian state space dynamics on the natural (log-ratio) parameters of the topic multinomials, and also on the mean of a logistic normal prior over per-document topic proportions. Exact posterior inference is intractable because the model is nonconjugate. The authors therefore derive variational approximations in which the variational parameters act as pseudo-observations, smoothed either by a Kalman filter or by nonparametric wavelet regression. They apply the model to OCR'ed articles of the journal Science from 1880 to 2000. DTM predicts next-year documents better than static LDA baselines, and it also gives an interpretable view of how scientific themes and vocabulary shifted.
Key points
- Time dynamics: Topic parameters follow a random walk, β_t,k ~ N(β_t−1,k, σ²I), and the proportion mean evolves the same way, α_t ~ N(α_t−1, δ²I). Topic correlation dynamics are left out.
- Inference: Two variational schemes are proposed, variational Kalman filtering and variational wavelet regression. The wavelet method is illustrated with the Haar basis, while the experiments use Daubechies wavelets. Document-level updates follow LDA-style coordinate ascent, and topic-level parameters are fit with conjugate gradient.
- Data and qualitative results: The corpus is the Science archive (1880–2000). A 20-topic model shows each topic's top words at ten-year intervals and traces how individual word probabilities change over time.
- Prediction experiment: For every 5th year from 1900 to 2000, the models were trained on all earlier articles and evaluated on the following year's documents using a variational upper bound on their negative log likelihood. DTM was compared against LDA-prev (previous year only) and LDA-all (all prior articles) and showed better predictive accuracy.
- Future work: The authors propose autoregressive drift terms, heteroscedastic noise, and birth–death or Galton-Watson processes so that topics can appear and disappear over time.
Cited by
- BERTopic: Neural topic modeling with a class-based TF-IDF procedure
- Diachronic word embeddings and semantic shifts: a survey
Cite
@inproceedings{blei2006dynamic,
title = {{Dynamic Topic Models}},
author = {Blei, David M. and Lafferty, John D.},
booktitle = {ICML 2006},
year = {2006},
doi = {10.1145/1143844.1143859},
url = {https://doi.org/10.1145/1143844.1143859}
}References
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