Attribute control charts are dedicated to the monitoring and control of counting type data in the form of proportions or direct counts.
A p chart is used to monitor sample proportions ˆp. However, when monitoring counts over continuous intervals of time, area, or volume and there is no definite limit on the number of counts that can be observed, a c chart is considered.
The interpretation of p and c charts is very similar to that of variable control charts. The out-of-control signals are also the same.