1. A method of generating a decision graph from event stream data, wherein the event stream data includes a plurality of events, and each event includes an associated time stamp, the method comprising: generating decision nodes for the graph, wherein the decision nodes each comprise a question having a temporal element; generating leaf nodes for the graph, wherein the leaf nodes each comprise a rate parameter; and iteratively splitting and merging nodes in the graph in order to maximize a measure of purity of outcomes in resulting nodes.