Probabilistic latent semantic indexing
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In this paper, we investigate the text classification and regression problems: given a corpus of text documents as training, each of which has a response label, the task is to train a predictor for predicting its response of any given document. In previous work, many researchers decompose this task into two separate steps: they first use a generative latent topic model to learn low-dimensional semantic representations of documents; and then train a max-margin predictor using them as features. In this work we demonstrate that it is beneficial to combine both steps of learning low-dimensional representations and training a predictor into one step of minimizing a singe learning objective. We present a novel step-wise convex optimization algorithm which solves this objective properly via a tight variational upper bound. We conduct an extensive experimental study on public available movie review and 20 Newsgroups datasets. Experimental results show that compared with state of art results in the literature, our one step approach can train noticeably better predictors and discover much lower-dimensional representations: a 2% relative accuracy improvement and a 95% relative number of dimensions reduction in the classification task on the Newsgroups dataset; and a 5.7% relative predictive R2 improvement and a 55% relative number of dimensions reduction in the regression task on the movie review dataset.