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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





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





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


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