A u-chart is an attributes control chart used with data collected in subgroups of varying sizes. U-charts show how the process, measured by the number of nonconformities per item or group of items, changes over time. Nonconformities are defects or occurrences found in the sampled subgroup. They can be described as any characteristic that is present but should not be, or any characteristic that is not present but should be. For example, a scratch, dent, bubble, blemish, missing button, and a tear are all nonconformities. U-charts are used to determine if the process is stable and predictable, as well as to monitor the effects of process improvement theories.
A u-chart shows process changes over time by looking at the number of nonconformities produced by the process. The plotted values represent a ratio. The subgroup size can vary, so control limits can adjust to reflect this, just as p-charts do.
U-charts should be used over p-charts when the number that is being counted can be larger than the number you are inspected. For example, if each stain is counted on an individual piece of clothing, or each scratch on a single door panel rather than just noting the entire unit as stained, or scratched respectively.
When to use classification (attributes) control charts:
1. When you want to see if your process is stable and predictable;
2. When you want to see how a planned change affects the process;
3. When the data you are examining represents counts of some type;
4. When the time order of the data is preserved.
This chart is part of PI VIZpack™ by PQ Systems, a collection of eight process improvement charts for Power BI®. They provide a clear way to visualize and evaluate process behavior and performance. Get started with a free two-week trial today. After the trial, you may continue using the basic features or upgrade to the premium version. To learn more visit our website.