Pp (process performance) is considered the ‘long-term’ potential capability measure. Contrast this with Cp (capability of the process) which is more of a ‘look backwards’ of where the process capability has been.
Both Cp and Pp are comparing the width of the voice of the process with the width of the voice of the customer. The voice of the customer is on top (the numerator) of the division equation. The voice of the process is on the bottom (in the denominator). What is being evaluated with the Cp/Pp index is the fatness (dispersion) of the process variation as it is compared to the specification window (i.e., the upper spec minus the lower spec). The two indices are a larger-is-better characteristic where values greater than 1.5 are generally acceptable.
Use: In some industries, competition dictates Cp and Pp indices far higher than 1.5. An example of an industry that would require a high Cp and Pp index is the smart phone industry. Smart phones today are measured in gigabytes of storage. One gigabyte is 1 billion bytes. If a manufacturer of a smart phone was satisfied with a Cp/Pp of 1.5 meaning that there would be 3.4 expected defects per 1 million memory locations. They would be out of business. In other industries, 1.5 is unrealistic. It really depends on competition.
