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:

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

• p-chart