A measure of process performance which takes into account the centering of the process. Ppk is calculated by dividing the distance of the process mean to the nearest specification, by 3 times the process standard deviation. If the process is centered exactly in the middle of the tolerance range, Ppk is equal to Pp.
The formula to calculate Ppk is shown, where USL and LSL are the upper and lower specification limits respectively and the process standard deviation 'scalculated' is obtained directly from the data.
Consider this example:
The Lower Specification Limit is 48
The Upper Specification Limit is 60
The process mean is 57
The calculated standard deviation is 1.65, giving a 3-Sigma process spread of 3*1.65 = 4.95.
Therefore, Ppk is is the minimum of: (57-48)/4.95 = 1.82 and (60-57)/4.95 = 0.61.
So Ppk is 0.61.
Pp, Cp, Cpk