The two types of Rejects charts are the p chart and the np chart. The name of the p chart stands for the Percentage or Proportion of rejects in a subgroup. The name of the np chart stands for the Number of rejects within a p-type chart. You can also remember it as “not percentage” or “not proportional”.
A mnemonic to remember that the p chart and its partner the np chart represents Rejects data is to think of P as a “pea” and a canning plant which is rejecting cans of peas if they are not 100% acceptable. As p and np are a team, you should be able to recall this with the same story.
np Chart – Number of Rejects Chart for Constant Subgroup Size
An np chart is an attribute control chart that displays changes in the number of defective products, rejects or unacceptable outcomes. It is an indicator of the consistency and predictability of the level of defects in the process.
The np chart is only valid as long as your data are collected in subgroups that are the same size. When you have a variable subgroup size, a p chart should be used.