TECHNOLOGY OVERVIEW
# Percent P (Percent of Defectives) Chart

Percent Charts are used for the same applications that P Charts are used i.e. to control the fraction of non-conforming products, although they can be used to also control fraction conforming (yield).

The difference is simply that proportion defectives has been replaced with percent defectives obtained by multiplying proportion defectives by 100.

As with P Charts Control limits are based on the binomial distribution.

Hence

LCL=100*(p – 3 * sqrt(P((1-p)/sample size))) =0 if negative

UCL=100*( p + 3 * sqrt(P((1-Np)/sample size)))

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

LCL= target – 300 * sqrt(P((1-p)/sample size))

UCL= target + 300 * sqrt(P((1-p)/sample size))

N is the sample size

P is the process proportion defectives or an estimate if not available. P=Percent Defective/100

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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