Summary

c- Charts

While XmR charts are the most often applied in organizations, and are the most appropriate charts to use most often, they are not infallible. 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 each other.

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 centerline. 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 question to ask yourself is: "Why am I charting rare events? What do I hope to discover?" Charting the number of times a rare event occurs is pretty useless. However, 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. Convert these numbers into the average moving ranges and, voil  , you can build an XmR chart.