Np Chart

In this chart, we plot the number of defectives (per batch, per day, per machine) as in the C chart. However, the control limits in this chart are not based on the distribution of rare events, but rather on the binomial distribution. Therefore, this chart should be used if the occurrence of defectives is not rare (e.g., they occur in more than 5% of the units inspected). For example, we can use this chart to control the number of units produced with minor flaws.

The Np chart is described in most standard quality control textbooks; see for example, Hart and Hart (1989), Juran (1962), Montgomery (1985, 1991), or Vaughn (1974).