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Interpreting an X-bar
/ R Chart
Always
look at the Range chart first. The control limits on the X-bar chart are
derived from the average range, so if the Range chart is out of control,
then the control limits on the X-bar chart are meaningless.
After reviewing the Range
chart, interpret the points on the X-bar chart relative to the control limits
and Run Tests. Never consider the points on the X-bar chart relative to
specifications, since the observations from the process vary much more than
the subgroup averages.
Interpreting the Range Chart
On the Range 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 those
process elements that contribute to sporadic changes in variation. To use
the data you have, turn Auto Drop ON, which will remove the statistical
bias of the out of control points by dropping them from the calculations
of the average Range, Range control limits, average X-bar and X-bar control
limits.
Also on the range chart,
there should be more than five distinct values plotted, and no one value
should appear more than 25% of the time. If there are values 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 chart, look at the X-bar Chart.
Interpreting the X-bar Chart
After reviewing the Range
chart, look for out of control points on the X-bar Chart. If there are any,
then the special causes must be eliminated. Brainstorm and conduct Designed
Experiments to find those process elements that contribute to sporadic changes
in process location. To use the data you have, turn Auto Drop ON, which
will remove the statistical bias of the out of control points by dropping
them from the calculations of the average X-bar and X-bar control limits.
Look for obviously non-random
behavior. Turn on the Run Tests, which apply statistical tests for trends
to the plotted points.
If the process shows control
relative to the statistical limits and Run Tests for a sufficient period
of time (long enough to see all potential special causes), then we can analyze
its capability relative to requirements. Capability is only meaningful when
the process is stable, since we cannot predict the outcome of an unstable
process.
See also:
Interpreting
the Run Tests
SPC
Concepts
When
to Use an X-bar / R Chart
X
Bar Chart Calculations
Range
Chart Calculations
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