np Chart in Excel
The purpose of an np chart is to evaluate process stability when counting the fraction defective.
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 100 or errors per each sample of 50 invoices.
np chart data has a constant sample size and looks like this:
Example of an np chart created using the QI Macros SPC Software 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?