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 , where n is the sample size and 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, , is constant.
See also p-chart
References