EWMA Chart in Excel
Use the EWMA Chart when you have one sample and want to detect small shifts in performance.
The EWMA (exponentially weighted moving average) chart's performance is similar to the Cusum chart.
Example of an EWMA Chart created in the QI Macros for Excel
To create an EWMA control chart within the QI Macros:
- Highlight your data and select "EWMA" from the "Control Charts (SPC)" drop-down menu (we offer an EWMA fill-in-the-blank template, as well).
- Once selected, you will be prompted to either accept the default alpha parameter of 0.2 or enter in your own:
- After you have created your chart, you can update/edit your alpha parameter, under the "Obs 1 Data" tab, in the "Weight" cell:
Per Montgomery 4th Edition, “values of λ in the interval 0.05 ≤ λ ≤ work well in practice, with λ = 0.05, λ = 0.10, and λ = 0.20 being popular choices. A good rule of thumb is to use smaller values of λ to detect smaller shifts.”
Note: The lower the value of the alpha parameter, the closer your UCL and LCL will be to the CL; and vice versa.