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Pp
Index
A Process Performance Index.
The ratio of the tolerance (specification, or permitted amount of variation)
to the variation in a sample.
- A value of 1 indicates
the sample variation exactly equals the tolerance.
- Values of less
than 1 indicate the allowable variation (the tolerance) is less than
the sample variation.
- Values of more
than 1 indicate that the sample variation is less than the tolerance.
- Use of Performance
Indices is generally discouraged in favor of Capability Indices wherever
possible.
Normal distributions:
Best Fit (Johnson) and
True-Position (Rayleigh) distributions:
Folded Normal distributions:
where:
 x
is Sample Sigma .
Ordinate0.99865 and ordinate0.00135
are the z-values of the non-normal cumulative distribution curve at the
99.865 percentage point and the 0.135 percentage points, respectively.
See also:
Process
Capability Charts
Ppk
Pr
Ppm
Process
Performance
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