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A u-chart is a type of control chart used to monitor the average number of nonconformities per unit when measuring subgroups at regular intervals from a process.
A u-chart is a scaled version of the c-chart representing the average number of nonconformities per unit rather than the number of nonconformities. The same assumptions and recommendations apply.
For data with different subgroup sizes, the control limits vary although the center line is constant. A standardized version of the control chart plots the points in standard deviation units. Such a control chart has a constant center line at 0, and upper and lower control limits of -3 and +3 respectively making it easier to see patterns.