Number Defective (np)#
The following table lists the properties of the Number Defective (np) chart.
| Property | Description |
|---|---|
| Chart | Number Defective (np) |
| Process observation types | Attributes |
| Process observations relationships | Independent |
| Sample Size | Constant, usually >=50 |
| Distribution type | Binomial |
| Size of shift to detect | Large (≥ 1.5σ) |
| Number Defective (Indicator 1) | \(p=number\;of\;rejects\) |
| Number Defective (Indicator 2) | Not applicable |
| Standard Deviation for sample | \(s = \sqrt{n\overline{p}\left ( 1-\frac{n\overline{p}}{n} \right )}\) Where \(n\overline{p}\) is the centerline and \(n\) is the sample size which is constant |
| Number Defective (Indicator 1) Centerline | \(n\overline{p}=\frac{total\;number\;of\;rejects}{number\;of\;samples}\) |
| Number Defective (Indicator 1) Control Limits | \(UCL = n\overline{p} + 3s\) \(LCL = max(n\overline{p}-3s;0)\) |
| Indicator 2 Centerline | Not applicable |
| Indicator 2 Control Limits | Not applicable |
Table: Number Defective chart properties