np Chart in Excel
Use an np chart when counting the fraction defective over a constant sample size
The np chart is useful when it's easy to count the number of defective items and the sample size is always the same. Examples might include: the number of defective circuit boards per 1000 or incorrect invoices per each sample of 50.
np chart data with a constant sample size looks like this:
To create an np chart in Excel using QI Macros:
- Just click and drag over the data to select it.
- Then click on the QI Macros menu, Control Charts, then np chart.
- QI Macros will do all the math and draw the np chart for you. QI Macros will also turn any unstable points or conditions on the np chart red. These should be investigated.
Example of an np chart created using QI Macros for Excel
A fully capable process delivers zero defects. Although this may be difficult to achieve, it should still be our goal. Once we resolve the out-of-control point, we could use the quality problem solving process to begin to eliminate the common causes of defects. What are the most common types of defects? Why do they occur? What are the root causes of these defects?