Further to what you might have read in http://qualitynotion.com/what-control-chart-shall-i-use-3/..

Unlike the C chart, U chart does not require a constant number of sample items, and it does not require any limit to the potential number of defects/nonconformities. Further more for a P- chart of NP chart, the number of non conformities cannot exceed the number of items on a sample, but for a U chart it is acceptable because what is being addressed is not the number of defective items, but the number of defects on the sample. In short,

Attribute Data  charts

 

 

 

 

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