The NP Chart is a control chart of number of defectives. It is used to track the number of defectives and detect signs of special causes.
1. Choose ProcessMA > Control Charts > Attribute Charts > NP.
2. In Variable, select the column containing the data.
· For constant subgroup size:
- Check Constant size.
- In Subgroup, enter the subgroup size.
· For non-constant subgroup size:
- Uncheck Constant size.
- In Subgroup, select the column containing the subgroup codes.
3. Click OK.
Optional
4. In Proportion, enter the population proportion, otherwise it will be estimated from the sample.
5. In Control limit multiples, enter the position to draw the upper and lower control limits. Multiple entries must be separated by commas (,).
6. In Lower limit bound, enter the bound on the lower control limit.
7. In Upper limit bound, enter the bound on the upper control limit.
8. In Phase, select the column containing the phase codes. (Phase can useful to compare groups of data like before and after improvement)
Note To select a column of data into a textbox, double-click on any of the column names shown in the list on the left of the dialog box while in the textbox.
Variable: Numeric.
Subgroup (For non-constant subgroup size): Text or Numeric; Must contain equal number of data points as the Variable.
Subgroup (For constant subgroup size): Integer; Must be at least 2.
Proportion: Numeric; Between 0 to 1.
Control limit multiples: Numeric.
Lower limit bound: Numeric.
Upper limit bound: Numeric.
Phase: Text or Numeric; Must contain equal number of data points as the Variable.
You work in a paper manufacturing plant and you need the control the quality of the paper. For each batch, you inspect a certain quantity to make sure that the papers are ‘white enough’ for quality control. You want to use the NP chart to help you monitor.
1. Open worksheet ProcessMA > Tools > Data Files > Control.xls.
2. Choose ProcessMA > Control Charts > Attribute Charts > NP.
3. In Variable, select I – Stained.
4. In Subgroup, select J – Sampled.
5. Click OK.
Interpretation
Sample 20 is outside the control limit and you may want to investigate that production batch for quality issues.