Question: Why Use 25 Data Points for the Average & Limits in the Process Behavior Charts Template?

A question:

I have a question about the formulas for columns C & E in your template. Is there a reason they’re set to only calculate the average on the first 25 data points as opposed to all them?

My answer: Thanks for your question. It’s somewhat arbitrary, but 25 data points is good for a baseline. You certainly can use more data points, but only if it’s part of the same predictable system (with no shifts in performance).

There are diminishing returns from using more than 25 data points in the baseline.

I’d encourage you to experiment with a data set that’s stable and predictable over time. Try 10, 15, 20, 25, 30 data points for the baseline average and limits and see how much the “voice of the process” changes (if at all).

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