What is the difference between Cp and Cpk?
Cp accounts only for process variability
Cp = (upper specification limit - lower specification limit)/6*Standard Deviation
Cpk accounts for process variability and centering of the process mean to the design nominal.ie it considers the shift.
Cpk = Min (Cpl, Cpu); where:
- Cpl = (mu - LSL) / 3*Standard Deviation
- Cpu = (USL - mu) / 3*Standard Deviation