It Is Time To Get Rid Of Action Limits

Action limits cause you to tamper with a process and make it worse.

The following words of an anonymous poet as he (or she) immortalized the lessons from Deming’s funnel experiment.

“Tamper, tamper is the game, try to make all the same. Squeak and tweak it every day, off we go to the Milky Way.”

—Anonymous

I offer a corollary that may help understand the underlying problem:

“We love to tamper with our processes. We cannot help it. We are hard-wired to tamper with our processes!”

For many years, I have examined in-house and customer-generated specifications. Often, I have seen something curious called “action limits.” Action limits always fall somewhere between the target value and upper and lower specification limits.

The following is an example:

Brightness Specification
Lower Spec.

…

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Comments

Labeling Action Limits

This is interesting...I've run across the same thing. I've even know people to have rules for their action limits...three points in a row trending in the direction of the action limit triggered action. Did action limits come from ECP? Most of the people I've seen use them made that claim, anyway.
We should require anyone who wants to use them to label them in some meaningful way..."Tampering Limits" might help, if they understood tampering. Maybe just "act above this line to really screw the system up" or "abandon hope, all ye who enter here..."

Right on!

I've experienced non-statistical, Action Limit triggers with no defined subsequent process too, in my work as a Quality Technical Writer/ISO 9001&14001 Internal Auditor. Generally speaking, it would only make sense to have Action Limts if they were synonomous with the calculated process Control Limits on a control chart AND the action to be taken was pre-defined, well documented, and consistently followed (e.g., Out-of-limit data point trigger has specified person/s leading the investigation, analysis to be conducted, due date for initial findings, corrective/preventive action plan). Good job with your insight and explanations!

Action Limis

Not that I've ever used them but I thougt they were the same as "Warning Limits" (2 sigma limits). A British convention I think.

Rich DeRoeck
ex-tweaker

"Warning Limits"

"Warning" of what? That the process is operating with common cause variation? Labelling the 2 sigma limits as anything other than "2 sigma limit" is a demonstration of non-statistical thinking and will lead to tampering.