The upper range (or upper control limit,
or upper natural limit) is computed by multiplying the average moving range
(the center line of the mR chart) by 3.27.
For the X (individual values) chart. The upper range for the X chart is computed
by multiplying the average moving range of the associated chart by
2.66 and then adding the value for the center line of the X chart. The lower
range for the X chart is computed by multiplying the average moving range
by 2.66 and then subtracting the value for the center line of the X chart.
Notice that values for both representations (individual values and average moving
range values) must be gathered and computed. The upper and lower limits for
the individual values chart (X chart) depend on the average variations calculated
for the center line of the average moving range chart. Therefore, these charts are
interdependent and can be used to show relationships between the two types of
charts and the two types of data.
When you present these charts to management, they may decide that the variation
looks a little too big for their comfort.
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