(1) A chart used to monitor the number of defects in sequential production lots.
(2) An attributes control chart that is used to monitor the number of nonconformities per unit, such as defects per item. The subgroup size may vary.
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. Examples of processes suitable for monitoring with a u-chart include:
* Monitoring the number of nonconformities per lot of raw material received where the lot size varies
* Monitoring the number of new infections in a hospital per day
* Monitoring the number of accidents for delivery trucks per day
As with the c-chart, the Poisson distribution is the basis for the chart and requires the same assumptions.