NP Chart with Minitab

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

Data Point:

NP Chart EQ1

Center Line:

NP Chart EQ2

Control Limits:

NP Chart EQ3


  • 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.

About Michael Parker

Michael Parker is the President and CEO of the Lean Sigma Corporation, a management consulting firm and online six sigma training, certification, and courseware provider. Michael has over 25 years of experience leading and executing lean six sigma programs and projects. As a Fortune 50 senior executive, Michael led oversight of project portfolios as large as 150 concurrent projects exceeding $100 million in annual capital expenditures. Michael has also managed multi-site operations with the accountability of over 250 quality assurance managers, analysts, and consultants. He is an economist by education, earning his Bachelor of Science degree from Radford University while also lettering four years as an NCAA Division I scholarship athlete. Michael earned his Six Sigma Master Black Belt certification from Bank of America and his Black Belt certification from R.R. Donnelley & Sons.

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