Use a C chart to monitor the number of defects where each item
can have multiple defects.
You should use a C chart only when your subgroup sizes are equal.
If your subgroup sizes are not equal, use the U chart.
For example, a quality engineer for a wallpaper manufacturer wants to assess
the stability of the printing process. Every hour, the engineer takes a sample
of 100 feet of wallpaper and counts the number of printing defects, which
include print smears, pattern distortions, and missing ink. To see an example,
Example of C Chart.
You must be able to count the number of defects on each item or unit.
Subgroup sizes should be equal, or nearly equal. For details, go to
Data considerations for C Chart.