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 QI Macros for Excel
To create an EWMA control chart using QI Macros:
- Highlight your data and select "EWMA" from the "Control Charts (SPC)" drop-down menu.
- Next you will be prompted to either accept the default alpha parameter of 0.2 or enter in your own:
EWMA Chart Formula
Note: 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."
The lower the value of the alpha parameter, the closer your UCL and LCL will be to the CL; and vice versa.
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