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





Read More