TECHNOLOGY OVERVIEW
U (Average Number of Defects per unit) Chart
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.