Np-chart
# Np-chart

In statistical quality control, the np-chart is a type of control chart used to monitor the number of nonconforming units in a sample. It is an adaptation of the p-chart and used in situations where personnel find it easier to interpret process performance in terms of concrete numbers of units rather than the somewhat more abstract proportion.

The np-chart differs from the p-chart in only the three following aspects:

* The control limits are $n{\bar {p}}\pm 3{\sqrt {n{\bar {p}}(1-{\bar {p}})}}$, where n is the sample size and ${\bar {p}}$ is the estimate of the long-term process mean established during control-chart setup.
* The number nonconforming (np), rather than the fraction nonconforming (p), is plotted against the control limits.
* The sample size, $n$, is constant.
