Use U Chart to monitor the number of defects per unit, where each item can have multiple defects. Use this control chart to monitor process stability over time so that you can identify and correct instabilities in a process.
For example, an LCD manufacturer wants to monitor the number of dead pixels on 17-inch LCD screens. Technicians record the number of dead pixels for each screen. Each subgroup has a different number of screens. The manufacturer uses 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 identify any special causes that contribute to the unusually high number of dead pixels.
Where to find this control chart
When to use an alternate control chart
If you can determine only whether each item is defective or nondefective, use P Chart to plot the proportion of defective items.