The data in the Quality Control sample data folder records the defect count in boxes of automobile support braces. A box of braces is one inspection unit. The number of boxes inspected (per day) is the subgroup sample size, which can vary. The u-chart in u-chart is monitoring the number of brace defects per subgroup sample size. The upper and lower bounds vary according to the number of units inspected.
Note: When you generate a u-chart, and select Capability, JMP launches the Poisson Fit in Distribution and gives a Poisson-specific capability analysis. To use the Capability feature, the unit sizes must be equal.
Select Help > Sample Data Library and open Quality Control/
Select Analyze > Quality and Process > Control Chart > U.
Select # defects and click Process.
Select Date and click Sample Label.
Select Unit size and click Unit Size.

Help created on 9/19/2017