U charts will help you evaluate process stability when there can be more than one defect per unit.
This kind of chart in particular is useful when you can't have a constant measure unit, that is, that the non-conformings varies with each subject therefore a ratio is necessary
Examples include defects per product, errors per invoice, patient falls in a hospital.
To calculate an u Chart you can use the following formulas
- UCL: u + 3√(u/n)
- Cl: u = Σ(ui)/Σ(ni)
- LCL: u - 3√(u/n)
Steps in Constructing a u Chart
1. Find the number of nonconformities, c(i) and the number of inspection units,
n(i), in each sample i. 2. Compute u(i)=c(i)/n(i) 3. Determine the centerline of the u chart 4. The u chart has individual control limits for each subgroup i. 5. Plot the centerline, ubar, the individual LCL's and UCL's, and
the process measurements, u(i).
6. Interpret the control chart.
== References ==