How it works
Cp shows how many times the ±3σ spread of the process fits into the tolerance band, while Cpk also accounts for how well the process is centred. If Cpk is much lower than Cp, the process is off-centre and adjusting the setting can gain a lot.
Formula
Cp = (USL − LSL) / 6σ Cpu = (USL − x̄) / 3σ Cpl = (x̄ − LSL) / 3σ Cpk = min(Cpu, Cpl)
A Cpk of 1.33 (about 63 ppm) is a common requirement; critical characteristics and automotive customers often ask for 1.67. With pasted data the sample standard deviation (n−1) is used; if the data spans a long period, the result is really Pp/Ppk. The ppm estimate assumes a normal distribution.
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