u-chart

In statistical quality control, the u-chart is a type of control chart used to monitor 'count'-type data where the sample size is greater than one, typically the average number of nonconformities per unit. The u-chart differs from the c-chart in that it accounts for the possibility that the number or size of inspection units for which nonconformities are to be counted may vary. Larger samples may be an economic necessity or may be necessary to increase the area of opportunity in order to track very low nonconformity levels.