Select the method or formula of your choice.
In This Topic
Plotted points
Minitab plots the number of defectives in each subgroup, 
.
Center line
If you do not specify a historical value for the process proportion (
), then Minitab uses the average proportion of defectives from your data, 
, to estimate 
:
The location of the center line for each subgroup, 
, is calculated as follows:
Notation
| Term | Description |
|---|---|
 | size of subgroup  |
 | process proportion |
Control limits
Lower control limit (LCL)
The LCL for each subgroup 
is the greater of the following:
Upper control limit (UCL)
The UCL for each subgroup 
is the lesser of the following:
Notation
| Term | Description |
|---|---|
 | size of subgroup  |
 | process proportion |
 | parameter for Test 1. The default is 3. (For more information go to Select tests for special causes for NP Chart.) |
