Disclosed herein is a method for use in detection of abnormal behavior of a group of a plurality of entities of a computer system. The method is arranged to be performed by a processing system and includes: creating a model of normal behavior of the group of entities; and determining, in accordance with the model of normal behavior of the group of entities, a parameter indicative of abnormal behavior of the group of entities. Also disclosed is an equivalent computer readable medium and anomalous behavior detection system.