Charts | p 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 p chart is used when we’re looking a the number of defectives and the subgroup size is variable. 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, but the sample size might not be the same amount each day. This is where p charts are useful.
Control Limits
Since the subgroup size is NOT constant, so the upper and lower control limits (UCL & LCL) are NOT constant, either. So, for every sample, there will be a separate upper and lower control limit. This results in having a narrower control limit the larger your sample size is. Remember, p charts represent a Binomial distribution. Therefore, the Binomial calculations apply.
The subgroup size for n should be over 50.
- Control Line (CL) = p =
total defectives/total inspected = Σpn/Σn - Upper Control Limit (UCL) =
p + 3√ (p(1 - p)⁄n) - Lower Control Limit (LCL) =
p - 3√ (p(1 - p)⁄n)