U Chart:

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. U-charts can be created using software programs like SQCpack and CHARTrunner.