c Chart is a control chart for monitoring the count of defects in inspection units of constant size, where multiple defects can occur per unit.

What Is a c Chart?

• Metric: Total count of defects (c) per inspection unit
• Requirement: Constant inspection unit size (area, length, volume)
• Distribution: Poisson distribution assumption
• Key Difference: Counts defects, not defective units

Why c Charts Matter

When products can have multiple defects (scratches on a panel, voids in a weld), c charts track total defect count rather than simple pass/fail.

c Chart Example (Solder defects per PCB):
Average defects per board: c̄ = 4.5

Center Line: c̄ = 4.5
UCL = c̄ + 3√c̄ = 4.5 + 3√4.5 = 10.9
LCL = c̄ - 3√c̄ = 4.5 - 6.4 = 0 (use 0)

Sample data: 5, 3, 4, 6, 12*, 4, 3, 5, 2, 4
                      ↑ Out of control (>10.9)

c Chart Applications:

• Defects per wafer in semiconductor fab
• Paint blemishes per car body panel
• Errors per 1000 lines of code
• Surface defects per square meter of film