EWMA Chart

Overview | How to | Example

 

 

Overview

The EWMA chart is a control chart of exponentially weighted moving averages (of individuals or subgroup means). Each point contains a weighted effect from all the previous points. It can be used to track small shifts away from the target.

Moving averages are averages of a number (default length of MA = 3) of consecutive observations or subgroup means. If your data is in subgroups, the process standard deviation is estimated using pooled standard deviation as default. If your data is individual observations (subgroup size = 1), the process standard deviation is estimated using average moving range with moving range equals to 2 as default.

 

 

How to

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

  1. Choose ProcessMA > Control Charts > Time-Weighted Charts > EWMA

  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 Weight, enter the weight for EWMA (Numeric, >0 & <1)

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

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

  8. 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)

  9. In Lower Limit Bound, enter the bound on the lower control limit (Optional, Numeric)

  10. In Upper Limit Bound, enter the bound on the upper control limit (Optional, Numeric)

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

  12. In Method (Subgroup>1), choose the method of estimating sigma when subgroup size is greater than 1

  13. In Method (Subgroup=1), choose the method of estimating sigma when subgroup size is equals to 1

  14. In Moving Range Length, enter the number of observations used to calculate the moving range (Integer, >=2)

  15. Click OK

 

 

Example

You collected cycle time (days) data for application approval. You want to monitor the process using a EWMA chart.

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

  2. Choose ProcessMA > Control Charts > Time-Weighted Charts > EWMA

  3. In Variable, select Y - Approval Time

  4. In Constant Subgroup Size, enter 1

  5. Click OK

 

Results & Interpretation

 

 

 

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