More and more powerful computer technology inspires people to investigate information hidden under huge amounts of documents. We are especially interested in documents with relative time order, which we also call document streams. Examples include TV news, forums, emails of company projects, call center telephone logs, etc. To get an insight into these document streams, first we need to detect the events among the document streams. We use a time-sensitive Dirichlet process mixture model to find the events in the document streams. A time sensitive Dirichlet process mixture model is a generative model, which allows a potentially infinite number of mixture components and uses a Dirichlet compound multinomial model to model the distribution of words in documents. We consider three different time sensitive Dirichlet process mixture models: an exponential decay kernel model, a polynomial decay function kernel Dirichlet process model and a sliding window kernel model. Experiments on the TDT2 dataset have shown that the time sensitive models performs 18-20 % better in terms of accuracy than the Dirichlet process mixture model. The sliding windows kernel and the polynomial kernel is more promising in detecting events. We use ThemeRiver to provide a visualization of the events along the time axis. With the help of ThemeRiver, people can easily get an overall picture of how different events evolve. Besides Themeriver, we investigate using top words as a high-level summarization of each event. Experiment results on TDT2 dataset suggests that the sliding window kernel is a better choice both in terms of capturing the trend of the events and expressibility.
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