c chart
**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