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Change Ratio [Guideline in Control Limit Update]

A

Apoch

#1
Hello All,

I'd like to check with you whether you are following a change ratio to serve as guideline in the need for control limit update.

I've seen the mention of change ratio in Intel's training material: pg 53-54 of source:

wenku.baidu.com/view/acb18969a45177232f60a29e.html

Change ratio is to be calculated through the following formula:

CR = abs-value {[CL(old)-CL(new)]/stdev}

A change ratio of > 1.5 is deemed to be a problem while > 1.0 is deemed to be potentially a problem.

Thoughts?
 
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Steve Prevette

Deming Disciple
Staff member
Super Moderator
#2
Seems like there is a circular logic problem. I don't change my control limits unless I have justification to, generally with a trending signal (such as a point outside the control limits). But this seems to suggest that I calculate new control limits, and then use the new control limit as justification to calculate the new control limits. Hmmmm.
 
D

Darius

#3
Interesting, I have seen some control charts with changes on control limits but not a word about how it was done.

I have done something similar but as a statistical test :mg:, It has been a topic with some experts, if control charts can be taken as a statistical test, many are against it, but I found it handy for change detection. This approach can be more fast as detection rule..., it may worth have a try, but a point to be addresed is how many points should be taken from the last point, because a change includes more variation and a change of mean, but if you take 10000 points the last few point showing a change may not affect the statistics in a noticeable way.

Any process changes, narrow control limits or not reflecting the same parameters as the process it self (mean and standard deviation) could result on many false alarms because the practitioner didn't saw the change and changed the control limits accordingly. Agree that Donald Wheeler said the same Steve said, sort of "recalculate only if you know something changed", but fixing it just make it sort of specs limits.
 

Steve Prevette

Deming Disciple
Staff member
Super Moderator
#4
Interesting, I have seen some control charts with changes on control limits but not a word about how it was done.
Here is a writeup on the logic I use: http :// www .efcog .org/wg/esh_es/Statistical_Process_Control/SPC_Trending_Primer/daytwo/20 Life Cycle of a Trend Day Two.pdf - DEAD LINK (404) UNLINKED

I am probably more willing to follow a statistical signal in rebaselining than Dr. Wheeler would, in the case of one can't figure out the reason in the process that the change occurred. Certainly it is preferable to BOTH have a SPC signal, and also knowledge that the process indeed did change.
 
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