np-chart
np-chart | |
---|---|
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 |
Performance | |
Size of shift to detect | ≥ 1.5σ |
Process variation chart | |
Not applicable | |
Process mean chart | |
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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:
- 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[]
References[]
- ^ Montgomery, Douglas (2005). Introduction to Statistical Quality Control. Hoboken, New Jersey: John Wiley & Sons, Inc. p. 279. ISBN 978-0-471-65631-9. OCLC 56729567. Archived from the original on 2008-06-20.
Categories:
- Quality control tools
- Statistical charts and diagrams