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Interpreting
a Moving Average Sigma Chart
Always look at the
Sigma chart first. The control limits on the Moving Average chart are derived
from the average sigma, so if the Sigma chart is out of control,
then the control limits on the Moving Average chart are meaningless.
On the Sigma chart,
look for out of control points. If there are any, then the special causes
must be eliminated. Brainstorm and conduct Designed Experiments.
Note that Auto Drop is not invoked for Moving Average charts.
After reviewing the Moving
Sigma chart, interpret the points on the Moving Average chart relative to
the control limits. Run Tests are never applied to a Moving Average chart,
since the plotted points are inherently dependent, containing common points.
Never consider the points on the Moving Average chart relative to specifications,
since the observations from the process vary much more than the Moving averages.
If the process shows control
relative to the statistical limits 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: Moving Average Chart Calculations
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