Sometimes an event
will occur that skews the norm; that is, a rare event way outside of the average has
occurred. When this happens, a c-chart is better used. A c-chart is used for rare
events that are independent of one another.
Statistical Process Control n 255
The formulas for c-charts are different from XmR charts. First, calculate the
average count of the rare occurrence over the total time period that the occurrence
happened. That number becomes the center line. The upper limit is calculated by
adding the average count to three times the square root of the average count. The
lower limit is calculated by subtracting the average count from three times the
square root of the average count.
The questions to ask yourself are, ???Why am I charting rare events? What do I
hope to discover???? Charting the number of times a rare event occurs is pretty useless.
But charting the time periods between recurring rare events can be used to
help predict when another rare event will occur. To do this, count the number of
times the rare event occurs (usually per day per year) and determine the intervals
between the rare events.
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