Before start learning about the concepts behind process capability, it is high recommended that you review the section “Process Control VS Process Capability”.
Process Capability Index, Cp:
- Process Capability is commonly measured in units of process standard deviations.
Cp is the relationship between the process standard deviation and the range between the upper and lower specification limits.
Cp = (USL - LSL) / 6 σx
The minimum acceptable value for Cp is considered to be 1.
Process Capability Index, Cpk:
- Many times people use the capability index, Cpk which relates the process mean to the nominal value of the specification.
ZUSL= (USL - μx) / σx , ZLSL = (LSL - μx) / σx
We select the minimum of these two values:
Zmin = min [ ZUSL , - ZLSL]
We find Cpk index by dividing this minimum value by 3.
Cpk = Zmin / 3
Commonly, for the process capability to be considered acceptable, Cpk should be ≥ 1.00.