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The p chart monitors the fraction or proportion of
defective or nonconforming units.
Each point on the
chart represents the sample ratio of defective units to the
total number inspected within each subgroup or time
interval.
The solid centerline is the average of the
sample proportions.
The two dashed lines are the upper control limit (UCL) and
the lower control limit (LCL). Note that the value of the
UCL is variable because the sample size varied by
collection period.
The value of the LCL
is near 0.0, so it's not visible because it's flush with the
horizontal axis. Control
limits reflect the expected amount of variation in the
number of defective units when only common causes of
variation are present.
This chart looks good because all the points are within the control limits and
there are no obvious patterns.
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p Chart |