Anomaly explanation using metadata

Anomaly detection is the well-studied task of identifying when data is atypical in some way with respect to its source. In this work, by contrast, we are interested in finding possible descriptions of what may be causing anomalies. We propose a new task, attaching semantics drawn from metadata to a portion of the anomalous examples from some data source. Such a partial description of the anomalous data in terms of the meta-data is useful both because it may help to explain what causes the identified anomalies, and also because it may help to identify the truly unusual examples that defy such simple categorization. This is especially significant when the data set is too large for a human analyst to inspect the anomalies manually. The challenge is that anomalies are, by definition, relatively rare, and so we are seeking to learn a precise characterization of a rare event. We examine algorithms for this task in a webcam domain, generating human-understandable explanations for a pixellevel characterization of anomalies. We find that using a recently proposed algorithm that prioritizes precision over recall, it is possible to attach good descriptions to a moderate fraction of the anomalies in webcam data so long as the data set is fairly large.