Xbar S Chart with Minitab

What is a Xbar S Chart?

The X-S chart (also called Xbar S chart) is a control chart for continuous data with a constant subgroup size greater than ten. The Xbar chart plots the average of a subgroup as a data point. The S chart plots the standard deviation within a subgroup as a data point.
The Xbar chart monitors the process mean and the S chart monitors the variability within subgroups. The Xbar is valid only if the S chart is in control. The underlying distribution of the Xbar-S chart is normal distribution.

Xbar Chart Equations

Xbar Chart

Data Point:

Xbar S Chart EQ1

Center Line:

Xbar S Chart EQ2

Control Limits:

Xbar S Chart EQ3


  • m is the subgroup size
  • k is the number of subgroups
  • A3 is a constant depending on the subgroup size

S Chart Equations

S chart

Data Point:

Xbar S Chart EQ4

Center Line:

Xbar S Chart EQ5

Upper Control Limit:

Xbar S Chart EQ6

Lower Control Limit:

Xbar S Chart EQ7


  • m is the subgroup size
  • k is the number of subgroups
  • B3 and B4 are constants depending on the subgroup size

Use Minitab to Plot Xbar-S Charts

Data File: “Xbar-S” tab in “Sample Data.xlsx”

Steps to plot Xbar-S charts in Minitab:

  1. Click Stat → Control Charts → Variables Charts for Subgroups → Xbar-S.
  2. A new window named “Xbar-S Chart” appears.
  3. Select “Measurement” by clicking the box below
    “All observations for a chart are in one column”
  4. Select “Subgroup ID” as the “Subgroup Sizes.”
  5. Click the button “Xbar-S Options” to open a window named “Xbar-S 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 “Xbar-S Chart Options.”
  9. Click “OK”.
  10. The Xbar-S charts appear in the newly-generated window.

Xbar-S Charts Diagnosis

Model summary: Since the S chart is in control, the Xbar chart is valid. In both charts, there are not any data points failing any tests for special causes (i.e., all the data points fall between the control limits and spread around the center line with a random pattern). We conclude that the process is in control.

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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