Variational inference for semiparametric Bayesian novelty detection in large datasets

Luca Benedetti1, Eric Boniardi2, Leonardo Chiani3,4, Jacopo Ghirri3,4, Marta Mastropietro3,4, Andrea Cappozzo1, Francesco Denti5
1MOX, Department of Mathematics, Politecnico di Milano, Milan, Italy
2MOX - Department of Mathematics, Politecnico di Milano, Milan, Italy
3Department of Electronics, Information, and Bioengineering, Politecnico di Milano, Milan, Italy
4RFF-CMCC European Institute on Economics and the Environment (EIEE), Fondazione Centro Euro-Mediterraneo sui Cambiamenti Climatici, Milan, Italy
5Department of Statistics, Università Cattolica del Sacro Cuore, Milan, Italy

Tóm tắt

After being trained on a fully-labeled training set, where the observations are grouped into a certain number of known classes, novelty detection methods aim to classify the instances of an unlabeled test set while allowing for the presence of previously unseen classes. These models are valuable in many areas, ranging from social network and food adulteration analyses to biology, where an evolving population may be present. In this paper, we focus on a two-stage Bayesian semiparametric novelty detector, also known as Brand, recently introduced in the literature. Leveraging on a model-based mixture representation, Brand allows clustering the test observations into known training terms or a single novelty term. Furthermore, the novelty term is modeled with a Dirichlet Process mixture model to flexibly capture any departure from the known patterns. Brand was originally estimated using MCMC schemes, which are prohibitively costly when applied to high-dimensional data. To scale up Brand applicability to large datasets, we propose to resort to a variational Bayes approach, providing an efficient algorithm for posterior approximation. We demonstrate a significant gain in efficiency and excellent classification performance with thorough simulation studies. Finally, to showcase its applicability, we perform a novelty detection analysis using the openly-available Statlog dataset, a large collection of satellite imaging spectra, to search for novel soil types.

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