What is the average p value used in the P chart?

The average p value is the proportion of items in your process that are defective, assuming you have collected enough samples to have a stable estimate. The center line of a P chart is drawn at the average p value. Thus, a P chart may initially serve to establish a baseline level of defectives as much as to ensure process stability.

The % Defective statistic used in capability analyses is the average p value multiplied by 100.

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