The c-chart monitors the process variations due to the fluctuations of defects per item or group of items. The c-chart is useful for the process engineer to know not just how many items are not conforming but how many defects there are per item. Knowing how many defects there are on a given part produced on a line might in some cases be as important as knowing how many parts are defective. Here, non-conformance must be distinguished from defective items since there can be several non-conformances on a single defective item.
The probability for a nonconformance to be found on an item, in this case follows a Poisson distribution.
If the sample size does not change and the defects on the items are fairly easy to count, the c-chart becomes an effective tool to monitor the quality of the production process
One of the premises for a c-chart was that the sample sizes had to be the same. The sample sizes can vary when the u-chart is being used to monitor the quality of the production process and the u-chart does not require any limit to the number of potential defects
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