|Originally proposed by||Walter A. Shewhart|
|Rational subgroup size||n > 1|
|Measurement type||Number nonconforming per unit|
|Quality characteristic type||Attributes data|
|Underlying distribution||Binomial distribution|
|Size of shift to detect||≥ 1.5σ|
|Process variation chart|
|Process mean 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 , 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.