Page 224 - Six Sigma Demystified

204 Six SigMa DemystifieD

Use Options\Parameters for predefined centerline and control limits.
Use Options\Estimate to omit subgroups from calculations.

Use Options\Stages to define control regions.
Use Options\Test to apply relevant run-test rules.

Excel

Using Green Belt XL Add-On
Use New Chart\C Chart.

Options:
Use Data Range to specify plotted subgroups.

Use Options\Title|Label to set the column for x-axis labels.
Use Options\Control Limits for predefined centerline and control limits, to omit
subgroups from calculations, and to define control regions.
Use the Options\Analysis\Auto Drop checkbox to automatically remove out-of-
control groups from the control-limit calculations (they are still displayed on
the chart).
Use Options\Run Test to apply relevant run-test rules.

Interpretation

The upper and lower control limits indicate the bounds of expected process

behavior. The fluctuation of the points between the control limits is due to the
variation that is intrinsic (built in) to the process. This variation is due to “com-
mon causes” that influence the process. Any points outside the control limits
can be attributed to a “special cause,” implying a shift in the process. When a
process is influenced by only common causes, it is stable and predictable.
If there are any out-of-control points, then special causes of variation must
be identified and eliminated. Brainstorm and conduct designed experiments to
find the process elements that contribute to sporadic changes in process loca-
tion. To predict the capability of the process after special causes have been
eliminated, remove the out-of-control points from the analysis. This has the
effect of removing the statistical bias of the out-of-control points by dropping
them from the calculations of the average and control limits.