pp - Assessing Performance as if the Process was on Target
pp is a performance measure which assesses variation but not location. pp is calculated as follows.
where
- USL is the Upper Specification Limit,
- LSL is the Lower Specification Limit, and
- s is the sample standard deviation from all combined data.
The difference between ppm and pp is the term which includes the deviation from target. If the observed process average was on target, ppm and pp would be identical.
Calculation of pp and making comparisons to ppm will yield an understanding of the loss from being off target. An additional diagnostic measure, %Off-Target, is also be calculated which assists in this understanding.