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Calculation detail for defective (p) Control Charts

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Previously we covered the calculation detail for variables Control Charts. This time it is the turn of the p and np attributes charts for defective items. In contrast to the variables charts, which need two charts, attributes charts only produce one chart.

Calculation for Proportion Defective (p) Control Chart

The p-chart ise used for defective items (as opposed to total defects) where the number of measurements in each plotted point (subgroup) is different. This is typically because the subgroup is based on an event such as a production run or time, such as an hour’s production. 

The calculation need to take into account the number of items and the number of these which are defective. The total number of defects in each item is not considered as, once defective at any level, a product cannot continue.

The control limits for the p-chart are calculated as in Fig. 1. Note that the control limits for the p-chart are not straight lines – they are different for each plotted point.

 Calculation for p-chart

Next time: Calculation detail for defective (np) Control Charts

This article first appeared in Quality World, the journal of the Chartered Quality Institute