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U ChartStat > Control Charts > Attributes Charts > U
Tracks the number of defects per unit sampled and detects the presence of special causes. Each entry in the worksheet column is the number of defects in a sample (or subgroup), assumed to come from a Poisson distribution with the parameter m, which is both the mean and the variance.
By default, Minitab estimates the process average number of defects, m, from the data. This value is the center line. Minitab also calculates the control limits using this value.
If your data exhibit overdispersion or underdispersion a Laney U' chart may more accurately distinguish between common cause variation and special cause variation.
For more information, see Control Charts Overview and Attributes Control Charts Overview.
Variables: Enter the columns that contain the number of defects for each subgroup.
Subgroup sizes: Enter a number or a column containing the subgroup sizes. If the subgroups are not equal, each control limit is not a single straight line but varies with the subgroup size. If the subgroup sizes do not vary much, you may want to force the control limits to be constant by specifying a fixed subgroup size using U Chart Options > Limits.