System and method for improved anomaly detection using relationship graphs

An anomaly-detection system that gathers information relating to the relationships between entities and represents these relationships in a graph that interconnects each pair of related entities. The graph may represent a computer network, in which each node corresponds to a respective device in the network and each edge between two nodes indicates that the devices represented by the nodes exchanged communication with one another in the past. the system monitors each of the entities in the graph, by continually computing a single-entity anomaly score (SEAS) for the entity. If the SEAS exceeds a first threshold the system generates an alert. Otherwise, the system checks whether the SEAS exceeds a second, lower threshold. If so, the system computes a subgraph anomaly score (SAS) for the entity's subgraph. If the SAS exceeds a SAS threshold, an alert is generated. By computing the SAS in this manner resources are conserved.