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Cpu and Cpl are the Cpk values calculated for both Z values.
Therefore, Cpu is:
For the example:
Cpl is:
From Cpu and Cpl, it is evident that the smaller value for the example is Cpu, which is the same value as Cpk. By comparing Cpu to Cpl, it is evident that the overall average is off center and closer to the upper specification than the lower specification. The larger the difference between the Cpu and Cpl, the more off center the process.
The above article is an excerpt from the “Operational definition” chapter of Practical Tools for Continuous Improvement Volume 2 Statistical Tools.