P (Proportion Defective) Chart

The BIS.Net Team BIS.Net Team

P Charts are used to control the fraction of non-conforming products, although they can be used to also control fraction conforming (yield).

Control limits are based on the binomial distribution. It follows that if the total proportion of defectives is equal to p then the standard deviation is equal to sqrt(P((1-p)/sample size))


LCL= p – 3 * sqrt(P((1-p)/sample size)) =0 if negative

UCL= p + 3 * sqrt(P((1-p)/sample size))

Control limits can be placed around a target i.e.

LCL= target – 3 * sqrt(P((1-p)/sample size))

UCL= target + 3 * sqrt(P((1-p)/sample size))

But this approach implies that it is OK to target a known level of defectives. The only target should, at least philosophically be zero defectives

If p is unknown p is replaced with an estimate of p

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