A process capability index C_p of 1.0 describes a production where the difference in the specification limits is equal to 6 standard errors.

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A process capability index (Cpk) of 1.0 indicates that the process is capable of producing within 3 standard deviations of the mean. Therefore, the difference in specification limits would be 6 standard deviations, which is equivalent to a value of 2 for the process capability index. A value of 1.0 would describe a process that is capable of producing within 1 standard deviation of the mean, which is not equal to the difference in specification limits being 6 standard errors.

A Process Capability Index Cp of 1

A Process Capability Index C_p of 1