Xbar-S Chart

Overview | How to | Example

 

 

Overview

The Xbar-S Chart consists of is a control chart of subgroup means and a control chart of subgroup standard deviations. It is used to track both the process level and process variation at the same time, and detect signs of special causes. The process standard deviation is estimated using pooled standard deviation as default.

 

 

How to

At the Excel Menu (For Excel 2007, go to Add-ins first)

  1. Choose ProcessMA > Control Charts > Variables Charts for Subgroups > XBar-S

  2. In Variable, select the column containing the data (Numeric)

  3. In Subgroup, select the column containing subgroup indicators (Optional, >=2 distinct values)

  4. In Constant Subgroup Size, enter the subgroup size if it is constant (Optional, Integer, >=2)

  5. In Historical Mean, enter the population proportion, otherwise it will be estimated from the sample (Optional, Numeric)

  6. In Historical Standard Deviation, enter the population standard deviation, otherwise it will be estimated from the data (Optional, Numeric, >0)

  7. In Control Limit Multiples, enter the position to draw the upper and lower control limits. Multiple entries must be separated by commas (Optional, Numeric, >0, Up to 3 multiples)

  8. In Xbar-Lower Limit Bound, enter the bound on the lower control limit for the Xbar chart (Optional, Numeric)

  9. In Xbar-Upper Limit Bound, enter the bound on the upper control limit for the Xbar chart (Optional, Numeric)

  10. In S-Lower Limit Bound, enter the bound on the lower control limit for the S chart (Optional, Numeric)

  11. In S-Upper Limit Bound, enter the bound on the upper control limit for the S chart (Optional, Numeric)

  12. In Phase, select the column containing the phase indicators (Optional, >=2 distinct values)

  13. In Method, choose the method of estimating sigma

  14. Check the various checkboxes to perform respective tests

  15. Click OK

 

 

Example

You are the branch manager of a bank and you need to ensure that time customers spent waiting in the queue is in control. For each day you took 10 measurements on the waiting time. You want to generate a Xbar-S Chart to track the process level and variation.

  1. Open data worksheet by choosing ProcessMA > Tools > Data

  2. Choose ProcessMA > Control Charts > Variables Charts for Subgroups > Xbar-S

  3. In Variable, select V - Waiting Time

  4. In Constant Subgroup Size, enter 10

  5. Click OK

 

Results & Interpretation

The fifth point of the Xbar Chart lies outside of the control limit suggesting a possible special cause. The points in the S Chart is random and within the control limit, suggesting that the variation of the process is stable.

 

 

Copyright © ProcessMA