The np chart is used when the data can only be whole numbers, as in counting, it is known discrete data (also known as attribute data). Every item in the sample is evaluated for only the number of defects, flaws, or occurrences, etc.
Use: The np chart is used with a fixed sample size where you are counting the number of defectives in the sample. If you remember, the difference between a defect and a defective is this. A defect can be found on an otherwise acceptable product; whereas, a defective means that the whole item is unacceptable. For example, you might have some minor defects in the fabrication of the windshield. The windshield passes inspection even though there are some minor defects. Defects are still undesirable, but there are an insufficient number of defects to render the windshield unusable. If, however, the windshield is cracked, the windshield would be rendered defective, and totally unusable. With the np chart, we are looking at defectives – not defects, and remember that were looking at a constant (as opposed to varying) sample size with the np chart. If it is a varying sample size and we are evaluating defectives, we would instead be using a p chart.
