So
the median range chart will automatically ???throw out??? the very high-end or lowend
values. Use of the median moving range approach is valid; however, the formulas
(constants) change. The constant value 2.66 changes to 3.14, and the constant
value 3.27 changes to 3.87. If you decide to use both methods on your data, you
will find some slight differences in the limits but not much. Try it and see.
Don??™t panic over computing these numbers. Most of the automated tools out
there will do it for you and draw the charts. While we make no recommendations,
the tool we used to generate the charts in this chapter is an add-on to Microsoft
Excel.
Hints on How to read the Charts
The most obvious interpretation is when one or more data points fall outside your
control limits (either upper or lower). Those values should be investigated for assignable
causes, and the assignable causes should be fixed. If your control chart shows
three out of four consecutive points hovering closer to the limits than to the center
line, this pattern may signal a shift or trend and should be investigated (because
predictable processes generally show 85 percent to 90 percent of the data closer to
the center line than to the limits).
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