Online Outlier Detection in Open Feature Spaces

Outlier detection is essential for data compliance, fraud prevention, and strategic decision-making. Finding outliers relies on study of feature space to find anomalous instances. As the feature dimension increases, it will inevitably complicate the process and hinder the models from finding genuine outliers. In this paper, we investigate an ever-more challenging task, online outlier detection (OOD) problem, where data points to be examined for outlier detection are characterized by two dynamic changes: (1) increasing volume instead of a static set; and (2) evolving feature space instead of a known set. Such instance and feature space dynamics impedes traditional OD techniques reliant on geometric data structure for distinguishing outliers. To aid, we propose a new approach coined Online Outlier Detection in Open Feature Spaces, which circumvents this limitation by learning a latent hypersphere representation, respectively positioning regular and anomalous data points inside and outside its boundary. The crux of our approach tailors a reconstruction loss, allowing each data point to be represented as an addition of its pertinent feature embeddings. Each of these embeddings is updated non-intrusively, championing both efficient and incremental learning of the latent hypersphere. Extensive experiments on twelve benchmark datasets underscore the robustness and superior performance of our method against seven leading counterparts.