Autocorrelation and special cause variation in the AIAG SPC manual

[Audit Monkey]

It cannot have been easy for them to try to boil down all the SPC material out there into one manual and in general they have done a good job. But you are right, they have made some errors. Have you read the definition for unimodal for example: "a distribution is said to be unimodal if it has only one mode."

 

[bobdoering]

It cannot have been easy for them to try to boil down all the SPC material out there into one manual and in general they have done a good job. But you are right, they have made some errors. Have you read the definition for unimodal for example: "a distribution is said to be unimodal if it has only one mode."

Hmmmmmm....deep.

 

[Jim Wynne]

It cannot have been easy for them to try to boil down all the SPC material out there into one manual and in general they have done a good job. But you are right, they have made some errors. Have you read the definition for unimodal for example: "a distribution is said to be unimodal if it has only one mode."

What's your definition?

 

[Tim Folkerts]

A large part of the problem is trying to force all different types of variation into just two categories ("common cause" vs "special cause"). In reality there are many sorts of variation. For some purposes, this simplistic dichotomy works, but for others it falls short (as we see here). Even throwing in "assignable" doesn't really cover it.

More precise language to more precisely describe the situations would be in order. So I humbly suggest the following might be more precise (and often overkill) for describing variation:

• Is the variation random in time and/or location? Then it is "uncorrelated" rather than "correlated".
• Does the variation apply to all parts? Then it is "common cause" rather than "special cause".
• Do you know what caused the variation? Then it is "assignable cause" rather than "unassignable cause".

So tool wear would be "correlated common assignable cause". A multi-cavity mold with one bad cavity would be "correlated special assignable cause". When my computer randomly freezes and needs to be rebooted, that is "uncorrelated special unassignable cause". An ideal process that is in control would have only "uncorrelated, unassignable, common causes".


Tim

 

[bobdoering]

An ideal process that is in control would have only "uncorrelated, unassignable, common causes".

I guess now we have to define what is an ideal process. I prefer:

A process in control is in the ideal state 100% conforming and predictable:
-must remain stable over time
-must operate in a stable and consistent manner
-must be set at the proper level
-the natural process spread must not exceed the product’s specified tolerance

I would be perfectly satisfied that a "correlated common assignable cause" process, such as grinding, would also meet those requirements, if controlled correctly.

Requiring an idea process to be "uncorrelated, unassignable, common causes" sure sounds sort of normalcentric to me.

 

[Tim Folkerts]

I guess now we have to define what is an ideal process. I prefer:

A process in control is in the ideal state 100% conforming and predictable:
-must remain stable over time
-must operate in a stable and consistent manner
-must be set at the proper level
-the natural process spread must not exceed the product’s specified tolerance

I would be perfectly satisfied that a "correlated common assignable cause" process, such as grinding, would also meet those requirements, if controlled correctly.

Requiring an idea process to be "uncorrelated, unassignable, common causes" sure sounds sort of normalcentric to me.

Good points. Perhaps I should have said a " typical idealized example of an in-control process" instead of "ideal". There are many examples of perfectly good (ie profitable) processes that do not follow this idealization.