Overview for U Chart

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.

Where to find this control chart

To create a U chart, choose Stat > Control Charts > Attributes Charts > U.

When to use an alternate control chart

  • If you can determine only whether each item is defective or nondefective, use P Chart or Laney P' Chart to plot the proportion of defective items, or use NP Chart to plot the number of defective items.
  • If your data exhibit overdispersion or underdispersion, Laney U' Chart may more accurately distinguish between common-cause variation and special-cause variation. Overdispersion can cause a traditional U chart to show an increased number of points outside the control limits. Underdispersion can cause a traditional U chart to show too few points outside of the control limits. The Laney U' chart adjusts for these conditions. You can test your data for overdispersion and underdispersion with U Chart Diagnostic. For more information, go to Overdispersion and underdispersion.