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The X-bar chart uses
sample means to monitor changes in the location of a
process.
On the X-bar chart, the sample means (or averages) are
plotted as points and connected with lines.
The solid centerline is
X-bar or the average of the sample means.
The two inner dashed
lines are the upper control limit (UCL) and the lower
control limit (LCL). The control limits reflect
the expected amount of variation in the sample means when
only common causes of variation are present.
The average is 4.9915, which is on target, but this plot
shows the process is out of control.
There are points outside the
control limits at collection periods 2, 25, and 35.
There is a trend of 6 increasing points from collection period 12 to 17.
Moreover, there are several other near-trends, which suggests
a cyclical pattern. Sure enough, the process peaks at collection
periods 7, 17, 25, and 35.
It bottoms out at periods 2, 12, 20, and 30.
Looking back at the records, you find that the valleys all occur during
the first shift, and the peaks occur during the third shift.
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X-bar Chart |