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Run
Test 7
by Paul
A. Keller, CQE, CQA
Run Test 7, discussed
in this month's column on Run Test Rules,
is a bit different from most of the other Run Test Rules. A brief description
of these rules, and their meaning, is also provided elsewhere in the Knowledge
Center: Interpreting
Run Test Rules.
The following X-Bar chart,
printed in Internet-friendly JPEG format from Quality America's SPC-PC
IV software, shows a process affected by Run Test 7: 15 Successive
Points within 1 Sigma of Centerline. The Zone Lines (sigma levels)
have been shown for convenience, but are generally turned off since SPC-PC
IV automatically checks for violations of each of the applied Run Tests
(or user-defined tests) each time a new subgroup is added. The control
limits are calculated, in this case, using all the plotted data with a
subgroup size of 3. Each of the circled subgroups in the chart are
in violation of Run Test 7.
At first glance, you might
suspect that the cause of this Run Test violation is a process improvement:
the process standard deviation was decreased. But, as is evident in the
Range chart, there is no reduction in the process variation.
What causes the points
on the X-Bar chart to hug the centerline?
This data, provided by
a major cosmetic manufacturer, represents the fill weight for bottles
of nail polish. The filling machine has 3 heads, so subgroups were conveniently
formed by taking a sample from each head.
The problem is that the
heads in the filling machine apparently have significantly different average
values. This variation between filling heads causes the within subgroup
variation (as plotted on the Range Chart) to be much larger than the variation
in the subgroup averages (represented graphically by the pattern of the
plotted points on the X-Bar Chart). The X-Bar Chart control limits, which
are calculated based on the Range Chart, are thus much wider than the
plotted subgroups.
The underlying problem
then is that the premise of a Rational
Subgroup has been violated: we tried to construct a subgroup out of
apples and oranges. But all is not lost (fruit salad isn't so bad): we've
learned something about our process. We've learned that the filler heads
are different, and that we could reduce overall variation by making them
more similar.
This type of multi-stream
behavior is not limited to cosmetic filling operations. Subgroups formed
from different locations on a warped fixture, different product lines
Consider the potential for irrational subgroups in these processes:
- A bank supervisor is
trying to reduce the wait time for key services. She constructs a control
chart, using subgroups based on a selection of 5 customers in the bank
at a time. Since she wants to include all the areas, she makes sure
to include loan applications as well as teller services in the subgroup.
- An operator finish
grinds 30 parts at a time in a single fixture. He measures 5 parts from
the fixture for his subgroup, always including the two end pieces. His
fixture is worn, so that the pieces on the two ends differ substantially.
Sound familiar? Send me
your stories (pak@qualityamerica.com)
and I'll include the best in a later column.
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