Use this test to determine whether a traditional U chart or a Laney U' chart is more appropriate to chart the number of defects per unit in your sample. If the test determines that your data exhibit overdispersion or underdispersion a Laney U' chart may more accurately distinguish between common cause variation and special cause variation.

The staff at a hospital chain records the number of medication errors each week. Examples of errors include delivering medication at the wrong time, delivering the wrong dose, and delivering the wrong medication.

The average subgroup size is 7500. Because of the large subgroup size, the staff wants to determine whether to use a Laney U' chart.

Data: MedicationErrors.MTW (available in the Sample Data folder).