Definition

A measure of process performance which takes into account the centering of the process. P_pk 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, P_pk is equal to P_p.

Examples

The formula to calculate P_pk is shown, where USL and LSL are the upper and lower specification limits respectively and the process standard deviation 's_calculated' 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, P_pk is is the minimum of: (57-48)/4.95 = 1.82 and (60-57)/4.95 = 0.61.
So P_pk is 0.61.

See Also

P_p, C_p, C_pk