Charts | pn Chart
Binomial Poisson Normal Charts Histograms Hypothesis Testing
p Chart pn Chart u Chart c Chart X-Bar R X-Bar s I-MR / X-MR Out-of-Control
A pn chart is used when we’re looking a the number of defectives and the subgroup size is constant. Here, the distribution is Binomial, since an item is either defective or it’s not. Two states! For instance, you may check a sample of your product daily and determine the number of defective items in that sample and the sample size you check is always the same, everyday. This is where pn charts are useful.
For a pn chart, the n is representative of the number of items inspected, while the p is representative of the proportion of defectives. Thus, if the number sampled is 100 and the proportion of defects is 0.02, then pn will be 2 defective items.
Control Limits
Since the subgroup size is constant, so the upper and lower control limits (UCL & LCL) are also constant. Remember, pn charts represent a Binomial distribution. Therefore, the Binomial calculations apply.
- Control Line (CL) = pn =
total defectives/total inspected = Σpn/Σn - Upper Control Limit (UCL) =
pn + 3√ pn(1 - p) - Lower Control Limit (LCL) =
pn - 3√ pn(1 - p)