1. A method for processing data associated with a data stream received by a data stream processing system, wherein the system comprises a plurality of processing elements, the method comprising the steps of: obtaining input data elements and output data elements associated with at least one processing element of the plurality of processing elements; and computing one or more intervals for the processing element using data representing observations of associations between inputs elements and output elements of the processing element, wherein, for a given one of the intervals, one or more particular input elements contained within the given interval are determined to have contributed to a particular output element.