C (Number of Defects) Chart

The BIS.Net Team 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.

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