A U chart plots the number of defects (also called nonconformities) per unit. It is possible for a unit to have one or more defects but still be acceptable in function and performance.
For example, you can use a U chart to monitor the following:
- The number of tears and pulls per 50 running feet of carpet
- The number of dead pixels per foot of LCD screen
You can inspect for one type of defect such as dead pixels. You can also inspect for several defects together such as dead pixels, stuck pixels, scratches, and blurry spots. An LCD screen may have 2 or 3 dead pixels, yet still be acceptable.
The C chart also plots defects. However, the C chart plots the number of defects per sample. The C chart is useful when the subgroup size is constant.
An LCD manufacturer wants to monitor defects on 17-inch LCD screens. Technicians record the number of dead pixels for each screen. Each subgroup has a different number of screens. They use a U chart to monitor the average number of dead pixels per screen.
Because of the unequal subgroup sizes, the control limits vary. On average, technicians find about 1 dead pixel on each screen. Subgroup 17 is out of control. The technicians should try to identify any special causes that may have contributed to the unusually high number of dead pixels.