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Conditional sampling of cylinder-pressure data is used to investigate cyclic variability in a premixed-charge spark-ignited engine operating under fuel-lean conditions. Unlike straight ensemble averaging of pressure data, conditional sampling applies a set of constraints to the pressure data such that like combustion events can be identified and grouped together. Ensemble averaging of pressure data from an engine that exhibits significant cycle-to-cycle variation is shown to produce a mean pressure history that is not representative of the combustion process. Conditional sampling provides a means of identifying and analyzing the different groups of pressure histories and therefore the different types of combustion processes that occur in an engine that exhibits cyclic variability.