3.9 Process Capability ( C _p And C _pk) And Process Performance ( P _p And P _pk) Indices

So far process performance that is evaluated through Sigma metrics has been discussed. The higher the Sigma value, the better a process is performing. Another method can be used to evaluate process capability and process performance. This method is also based on statistical measurements: C _p, C _pk, P _p, and P _pk. This section will provide a brief discussion of their definitions, interpretations, and calculations.

Process Capability ( C _P and C _Pk ) Index

Two key indices ( C _p and C _pk) are used related to technical process capability. They are seldom used in administrative service or transactional processes.

Capability

Capability is defined as the ability of a process to produce product/service within defined specification limits.

Process Potential Index ( C _P )

The process potential index measures the potential capability of a process. C _p is defined as the ratio of the allowable spread over the actual spread. This concept is presented in Figure 3.36:

C _p = (Allowable spread/Actual spread)

C _p = ( USL LSL)/6s

where:

LSL = Lower specification limit

USL = Upper specification limit

Figure 3.36: Conceptual Presentation of Cp

Actual spread is determined from the process data as collected and is calculated six...