U (Average Number of Defects per unit) Chart

The BIS.Net Team BIS.Net Team

U Charts are used to control the number of defects. Defects must not be confused with defectives. A defective part has one or more defects. In this instance, unlike the C Chart we are controlling the Average number of defects over 2 or more sample units.

For example if the number of cans of a beverage is 3, and 5 defects are found on the first can and 1 on the second and 3 on the third the average number of defects is (3+5+1)/3 =3

The control limits are

LCL=u - 3*sqrt(u/n)

UCL=u + 3*sqrt(u/n)

Control limits can also be placed around a target i.e.

LCL=target - 3*sqrt(u/n)

UCL=target + 3*sqrt(u/n)

U is the process average number of defects or an estimate if the process average number of defects is unavailable.

The factor of 3 is a standard control chart factor first proposed by Walter Shewhart, resulting in a low percentage of false alarms.

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