Methods and formulas for NP Chart

Select the method or formula of your choice.

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

TermDescription
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 calculated as follows:

Notation

TermDescription
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.)
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