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Cp vs Cpk

What is the difference between Cp and Cpk?


Cp
Cp accounts only for process variability

Formula:
Cp = (upper specification limit - lower specification limit)/6*Standard Deviation


Layer 1 Cp = 1 Cp > 1 Cp < 1 Upper Specification Limit (USL) Lower Specification Limit (LSL)

Cpk
Cpk accounts for process variability and centering of the process mean to the design nominal.ie it considers the shift.

Formula:
Cpk = Min (Cpl, Cpu); where:
  • Cpl = (mu - LSL) / 3*Standard Deviation
  • Cpu = (USL - mu) / 3*Standard Deviation

Layer 1 Cp = 1 / Cpk = 1 Cp > 1 / Cpk = 1 Cp = 1 / Cpk = 0 Upper Specification Limit (USL) Lower Specification Limit (LSL)

References
http://www.qualitygurus.com/courses/mod/forum/discuss.php?d=258(external link)







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