u Chart Formulas
u charts determine stability of "counted" data (e.g., errors per bill, dents in a car door, etc.) when there can be more than one defect per unit and the sample size varies.
Here is the formula used to calculate a u chart.
Conforms with ANSI/ASQC B1, B2, B3 1996
The u chart is just one of the tools included in the QI Macros for Excel SPC Software for Excel.