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Cp
index
A Process Capability Index.
The ratio of the tolerance (specification, or permitted amount of variation)
to the process variation.
- A value of 1 indicates the process variation exactly equals the tolerance.
- Values of less
than 1 indicate the allowable variation (the tolerance) is less than
the process variation.
- Values of more
than 1 indicate that the process variation is less than the tolerance.
- Values greater
than 1 are most desirable, and many customers recommend values of 1.33
or greater. This allows a margin of error in case a process shift occurs
that is not immediately detected.
Normal distributions:
Best Fit (Johnson) and
True-Position (Rayleigh) distributions:
Folded Normal distributions:
where:
 x
is Process Sigma,
Ordinate0.99865 and ordinate0.00135
are the 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
Confidence
Intervals for Cp
Cpk
Cr
Cpm
Process
Performance
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