Abstract: Systems and methods for classification using an explicit feature map or an approximate feature map based on a relative locality measure. In at least one embodiment, a method of authenticating a user operates data points having feature vectors pertaining to user events comprises selecting an approximate feature map based on a subset of features in each data point and a relative locality measure of a cluster including a plurality of the data points; mapping, to a feature space, the subset of features in each data point and a new data point pertaining to a phone call of the user using the selected approximate feature map; determining a classification of the new data point based on its relative locality measure with respect to a cluster in the feature space; storing the classification of the new data point in a memory device; and authenticating the user during the phone call.