TECHNOLOGY OVERVIEW
C (Number of Defects) Chart
BIS.Net Team
C 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 we are controlling the number of defects with a constant sample size.
For example the number of defects on a single beverage can, or 10 or 100 cans, as long as the sample size is constant.
The control limits are
Derived from the Poisson distribution
LCL=c - 3*sqrt(c)
UCL=c + 3*sqrt(c)
Control limits can be placed around a target i.e.
LCL=target - 3*sqrt(c)
UCL=target+ 3*sqrt(c)
C is the mean of the Poisson distribution estimated by the mean number of defectives
The factor of 3 is a standard control chart factor first proposed by Walter Shewhart, resulting in a low percentage of false alarms.