Question: What Should the MR-bar be?

This question also came from a webinar viewer:

“How do you tell if a process is “controlled” or not?

Is there a sort of max MR-bar to have in terms of % of the average or something?

I mean, if the MR-bar is 100 for a process “A” with a data average of 200 and the process “B” as also MR-bar=100 but with an average of 10,000.

Process B is much more “controlled” than “A.”

So is there a rule of thumb for that also? Like MR-bar has to be less than 20% of the average or something?”

My response:

You tell if a process is “controlled” (or “predictable”) when there are no signals in either the X Chart or the MR Chart. You can have a predictable process with WIDE limits and you can have a predictable process with narrow limits.
I don’t understand what you mean by the “Max MR-bar” — that’s the voice of the process and the MR-bar is what it is. If you don’t like the width of the Natural Process Limits (as calculated from the MR-bar) then you can work to improve the system in a way that will reduce variation. Reducing the MR-bar will lead to tighter limits on the X Chart. d

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