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Originally proposed by Walter A. Shewhart
Process observations
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
Not applicable
Process mean chart
Np control chart.svg
Center line
Control limits
Plotted statistic

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.[1]

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

  1. 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.
  2. The number nonconforming (np), rather than the fraction nonconforming (p), is plotted against the control limits.
  3. The sample size, , is constant.

See also[edit]