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