1. A system for comparing event data sets comprising: a calculating engine for calculating one or more derivatives for each event in a first and second set of events; a classifying engine for classifying each event by respective calculated derivatives; and a correlating engine for correlating first and second sets of events based on the classifications of the respective events, wherein each derivative is classified as a negative, zero or positive value and each event is classified according to a combination of negative, zero and positive values of the derivatives.