Page 346 - Six Sigma Demystified

326 Six SigMa DemystifieD

Process Capability 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 indi-
ces 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 options to indicate physical bounds that cannot possibly be exceeded
(e.g., zero is a lower boundary for cycle time data).

Interpretation

Compare the nonnormal and normal indices. Capability indices are quite sensi-
tive to assumptions of the distribution.
C provides the ratio of the tolerance (specification or permitted amount of
p
variation) to the process variation. A value of 1 indicates that the process variation

exactly equals the tolerance; values less than 1 indicate that the allowable variation
(the tolerance) is smaller than the process variation, an undesirable condition.
C provides a relative indication of how both process variation and location
pk
compare with the requirements.
When both C and C are available (i.e., bilateral specifications), then C
p pk p
can be used to estimate the best-case process capability (assuming that the
target value for the process is located exactly at the centerpoint between the
specification limits).
C is similar to the C index but takes into account variation between the
pm pk
process average and a target value. If the process average and the target are the
same value, C will be the same as C . If the average drifts from the target
pm pk
value, C will be less than C .
pk
pm