What is a u-Chart?

The Number of Defects per Unit (u) Chart is used to calculate and record the average number of defects found in the units of an inspection sample As its name suggests, the chart is used when you are interested in the defects per unit, rather than the overall number of defects in the sample.

The inspection unit could be:

• a single item (e.g a car)
• a group of items (e.g. 25 cars)
• a set of items (e.g. sets of wheel nuts)

The sample size (n) should always reflect the type of inspection unit. So, where the u-Chart is being used to measure the average number of defects found in the inspection of individual cars, n would be the number of cars in the sample.