Page 354 - Six Sigma Demystified

334 Six SigMa DemystifieD

Process Performance Index

Minitab

Use Stat\Quality Tools\Capability Analysis. Enter the upper and/or lower spec-
ifications. Use the Boundary option to indicate whether the specification is a
physical bound that cannot possibly be exceeded (e.g., zero is a lower boundary
for cycle time data).

Excel

Using Green Belt XL Add-On
Use New Chart\Histogram (Note: The histogram and resulting capability indices
also may be displayed as an option with the X and individual-X control charts.)
Enter the upper and/or lower specifications in the Analysis tab of step 3. If the
Johnson distribution is selected, use the Upper Bound and/or Lower Bound op-
tions to indicate physical bounds that cannot possibly be exceeded (e.g., zero is
a lower boundary for cycle time data).

Interpretation
Process performance uses sample sigma in its calculation, so it cannot provide
an estimate of process capability for the future. Since the process is not in a
state of control, we cannot predict its future performance.
The calculated process performance and process capability indices likely will

be quite similar when the process is in statistical control.

• A value of 1 indicates that the sample variation exactly equals the toler-
ance.

• Values less than 1 indicate that the allowable variation (the tolerance) is
less than the sample variation.

• Values greater than 1 are desirable, indicating that the sample variation is
less than the tolerance. Values greater than 1.3 or 1.5 are often recom-
mended.

Use of performance indices is generally discouraged in favor of capability
indices wherever possible.