NP Chart with JMP

March 23, 2016

What is NP Chart?

The NP chart is a control chart monitoring the count of defectives. It plots the number of defectives in one subgroup as a data point. The subgroup size of the NP-chart is constant. The underlying distribution of this control chart is binomial distribution.

NP Chart Equations

NP-chart
Data Point:

NP Chart EQ1

Center Line:

NP Chart EQ2

Control Limits:

NP Chart EQ3

Where:

  • n is the constant subgroup size
  • k is the number of subgroups
  • xi is the number of defectives in the ith subgroup

Use JMP to Plot an NP Chart

Data File: “NP.jmp”

Steps to plot a NP-chart in JMP:

  1. Analyze -> Quality & Process -> Control Chart -> NP
  2. Select Fail in the Process Field
  3. Select N in the Sample Size Field
  4. Click Ok

NP Chart Diagnosis

Model summary: Four data points, circled in red, fall beyond the upper control limit. We conclude that the NP chart is out of control.  Further investigation is needed to determine the special causes that triggered the unnatural pattern of the process.

 

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