Difference between cp and cpk

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Cp is a process capability index that measures the potential of a process to meet specifications, while Cpk measures the actual performance of the process.

Cp measures the spread of the process data around the target value, while Cpk measures the deviation of the process mean from the target value.
Cp only considers the upper and lower specification limits, while Cpk considers the mean of the process data as well.
Cp should be greater than or equal to 1 for a capable process, while Cpk should be greater than or equal to 1.33 for a highly capable process.
For example, if the specification limits for a process are 10 and 20, and the mean is 15 with a standard deviation of 2, then Cp would be (20-10)/(6) = 1.67 and Cpk would be min((20-15)/(3), (15-10)/(3)) = 1.

Difference between cp and cpk

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