1. A method for identifying repeating patterns in a sequential event stream, each event corresponding to an instance of an event type, comprising the steps of; creating a graph having two or more vertices, in which each vertex represents one of the types and pairs of vertices are linked by edges; moving a window through the event stream, identifying when events corresponding to a plurality of the types fall within the window at the same time and weighting the edges in the graph accordingly; and locating clusters within the edge-weighted graph, which correspond to repeating patterns in the event stream.