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Semiconductor manufacturing is a very complex process involving product mix, reentry lot flows, batching, and machine breakdowns all of which contribute to system variability. Reducing variability in a manufacturing process lowers lot cycle times. This study examines the issue of identifying machines that constrain the system and have a major impact on they system's cycle time. A new X-factor contribution measurement is developed that considers processing time variability and lot arrival variability among the constraining qualities of the machine groups. The effectiveness of this new measure to identify machine groups that significantly impact mean cycle time is tested in several operating scenarios.