U chart

From Elsmar Cove Quality Assurance and Business Standards Wiki

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.

See also

SPC Monitoring and Statistical Analysis Techniques Discussion Threads

C Chart

Np Chart

P Chart

References