P Control Chart: Nonconforming Proportion Online
What is a P Chart?
The P chart monitors the proportion of nonconforming units (nonconforming count divided by inspected count) and belongs to the attribute family of control charts. It models the process with the binomial distribution - each unit is judged conforming or not - and tracks the movement of the nonconforming proportion, suiting percentage-based quality management.
When to Use It
Use it when the data are attribute (go/no-go) and reported as a nonconforming rate, especially when sample sizes vary - for example when daily output differs or inspection counts change from lot to lot. When every batch is inspected at a fixed size, the NP chart (nonconforming count) is more intuitive.
P vs C and U Charts
The P chart tracks the proportion of nonconforming units (the fraction of defective products); the C chart tracks the number of defects on a fixed inspection unit; the U chart tracks the defect rate per unit when the unit size varies. P is about good/bad product while C/U count defects per product - different applications.
Control Limits
The center line is the average nonconforming proportion pbar; the limits are pbar +/- 3*sqrt(pbar*(1-pbar)/n), where n is the inspected quantity of each batch. Because n varies, the P chart limits form a stepped (sawtooth) band; the tool computes and plots the limits for each batch's actual sample size.