Usually, C charts are used to chart the total number of defects in a sample when the sample size is constant. 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 U chart also plots defects. However, the U chart plots the number of defects per unit. The U chart is useful when the subgroup size is not constant.
An LCD manufacturer wants to monitor defects on 17-inch LCD screens. Technicians record the number of dead pixels for each subgroup of 10 screens per hour. They use a C chart to monitor the number of dead pixels.
On average, technicians find 10 dead pixels in each sample. Sample 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.