Validity of Craig Hane's Global Local Index for X-bar R Chart Assessment

S

Sturmkind

#1
Hi!
Has anyone discussed the validity of Craig Hane's Global Local index for X-bar R chart assessment?

FYI G/L = 3σ(X-bar)/A2(R-bar). G/L~1 = OK, >>1 is Not OK.

Thanks!
 
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Miner

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#2
For those unfamiliar with this, see The Hidden Fallacy of Xbar and R Charts.

I think the concept of this index is valid, but I totally disagree with the direction that the author took it.

The ratio of the between subgroup variation to the within subgroup variation should approximate 1 (if Xbar and R charts are appropriate for the process). This I agree with, and it could be a useful diagnostic of your charts. However, an experienced person can tell the same thing visually.

At this point the author and I differ. The author advises that if GL >> 1, you should use the 3s limits of the subgroup means as control limits. Wrong and dangerous!

If GL does not approximate 1, it is evidence that your process is either truly out-of-control or that your subgroups are not rationally selected. Lets get specific:


  • GL >> 1: 1) Process may be out-of-control, or 2) Within subgroup variation is virtually 100% measurement variation. Solution: Change subgrouping; test for autocorrelation and, if necessary, use an IMR chart at a frequency beyond the period of autocorrelation.
  • GL << 1: Subgroups includes too much variation. Solution: Change subgrouping; check for mixed process streams and other sources of within subgroup variation.
The author's approach is to ignore the basic problem and apply a temporary fix. The better approach is to identify the root cause and address it with a permanent fix.
 
S

Sturmkind

#3
Thank you, Miner for your lucid reply! Your comments mirrored my thoughts exactly. Chart out-of-control conditions may be the least understood and most poorly applied practice to the science of applied statistics.

I am ever grateful to Eugene Grant & Richard Leavenworth for their excellent work 'Statistical Quality Control' for enhancing my understanding of visual graph interpretation and my grasp of the concept of the elusive standard deviation.

It has been my experience that control charts taken seriously have always lead to direct process improvement and have maximized the profit potential of the process to which they have been applied.
 
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C

Craig H.

#4
Thank you, Marc for your lucid reply! Your comments mirrored my thoughts exactly. Chart out-of-control conditions may be the least understood and most poorly applied practice to the science of applied statistics.

I am ever grateful to Eugene Grant & Richard Leavenworth for their excellent work 'Statistical Quality Control' for enhancing my understanding of visual graph interpretation and my grasp of the concept of the elusive standard deviation.

It has been my experience that control charts taken seriously have always lead to direct process improvement and have maximized the profit potential of the process to which they have been applied.
Yeah, Grant & Leavenworth's work is fantastic. I am bumping this thread because I would like to see more discussion. Interesting subject.
 
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