Page 431 - Six Sigma Demystified

Part 3 S i x S i g m a To o l S 411

Use 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
Always look at the range (or sigma) chart first. The control limits on the X-bar
chart are derived from the average range (or average sigma), so if the range (or
sigma) chart is out of control, then the control limits on the X-bar chart are
meaningless.

Interpreting the Range (or Sigma) Chart
On the range (or sigma) chart, look for out-of-control points. If there are any,
then the special causes must be eliminated. Brainstorm and conduct designed
experiments to find the process elements that contribute to sporadic changes
in variation. To predict the capability of the process after special causes have
been eliminated, you should remove the out-of-control points from the analy-
sis, which will remove the statistical bias of the out-of-control points by drop-
ping them from the calculations of the average range (or sigma), the range (or
sigma) control limits, and the X-bar control limits.
Also on the range (or sigma) chart, there should be more than five distinct
values plotted, and no one value should appear more than 25 percent of the
time. If values are repeated too often, then you have inadequate resolution of
your measurements, which will adversely affect your control limit calculations.

In this case, you’ll have to look at how you measure the variable and try to
measure it more precisely.
Once you’ve removed the effect of the out-of-control points from the range
(or sigma) chart, the X-bar chart can be analyzed.

Interpreting the X-Bar Chart
After reviewing the range (or sigma) chart, interpret the points on the X-bar
chart relative to the control limits and run-test rules. Never consider the points
on the X-bar chart relative to specifications because the observations from the
process vary much more than the subgroup averages.
If there are any out-of-control points on the X-bar chart, then the special
causes must be eliminated. Brainstorm and conduct designed experiments to