C Control Chart: Defect Counts on a Fixed Inspection Unit
What is a C Chart?
The C chart monitors the number of defects on each fixed inspection unit and is modeled with the Poisson distribution. Unlike P/NP charts, C counts defects rather than nonconforming units - a single product can carry several defects (e.g., three scratches on one housing).
When to Use It
Use it when the inspection unit size is fixed and the focus is defect counts: scratches per unit, defects per roll of fabric, blowholes per weld, pinholes per fixed plating area. If the inspection unit size varies, switch to the U chart.
C vs P and NP Charts
P/NP count nonconforming units (one count per product, a good/bad dichotomy); C/U count defects (one product can contribute multiple defects). When defects are sparse and density matters, C/U charts are more sensitive than nonconforming-rate charts.
Control Limits
The center line is the average defect count cbar (the mean of defect counts per unit); the limits are cbar +/- 3*sqrt(cbar), since the Poisson standard deviation is the square root of cbar. The tool calculates and plots automatically - no manual square roots needed.