NP Chart with Minitab

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What is a 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 Minitab to Plot an NP Chart

Data File: “NP” tab in “Sample Data.xlsx”

Steps to plot a NP chart in Minitab:

  1. Click Stat → Control Charts → Attributes Charts → NP.
  2. A new window named “NP Chart” appears.
  3. Select “Fail” as the “Variables.”
  4. Select “N” as the “Subgroup Sizes.”
  5. Click the button “NP ChartOptions” to open a window named “NP Chart Options”.
  6. Click the tab “Tests.”
  7. Select the item “Perform all tests for special causes” in the dropdown menu.
  8. Click “OK” in the window “NP Chart”
  9. Click “OK.”
  10. The NP chart appears in the newly-generated window.

NP Chart Diagnosis

Model summary: Four data points 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.