Work out process capability (cp cpk) instantly with clear inputs, formula shown and shareable results.
Cp compares the specification width with six standard deviations of process spread and assumes the process is perfectly centred. Cpk penalises off-centring by taking the worse of the two one-sided indices, so the gap between Cp and Cpk is exactly the capability you are losing to a mean that has drifted.
Cp
Cp = (USL - LSL) / (6 x sigma)
Cpk
Cpk = min[(USL - mean)/(3 sigma), (mean - LSL)/(3 sigma)]
1.33 is the usual minimum for an established process and 1.67 for safety-critical or new-product characteristics. Below 1.0 the process is producing out-of-specification parts.
Because Cpk uses the distance to the nearer specification limit. They are equal only when the mean sits exactly at the midpoint.