Pp

Pp

Capability index that compares process variation to the width of the spec limits:

The numerator is the width of the spec limits. The denominator is 6 standard deviations, which can be thought of as the width of the process. For the normal distribution 99.7% of values fall within +/-3 standard deviations of the average or into an interval 6 standard deviations wide. A Pp value of 1 means the process variation fills the spec limits. A Pp of 2 means the specs are twice as wide as the process. The larger Pp is, the better the capability.

Pp is similar to the capability index Cp. They are both calculated using the formula above. However, they use different methods for estimating the standard deviation. Pp uses the total standard deviation. Cp requires that the units be collected in subgroups over time and uses the within subgroup standard deviation, ignoring the effects of shifts between subgroups. Pp estimates actual performance, while Cp estimates the capability the process could achieve if made stable over time.

The acceptance criteria for a variable sampling plan can be stated in terms of the capability indexes Pp and Ppk.

Note: Pp does not depend on the average. Further Pp can only be calculated when there are both upper and lower spec limits.