Why is Cpk = lower of (USL-Xbar)/3sig & (Xbar-LSL)/3sig ?

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Bev D

Heretical Statistician
Staff member
Super Moderator
#12
Caster - i think you too lost my sarcasm. My examples were intended to point out the weaknesses and fallacies of the capability index.

I do keep my customer in mind. I constantly talk to my customers to understand their true needs and to educate them (back when I had OEM customers - now my customers are teh end users they only demand that the product work.) I do this by NOT using process capability indexes but by plotting my data and looking at it to know what my capability is. I do this so I understand how to inspect it and to prioritze improvement actions. I use these plots to begin the diagnostics in the search for root cause.

My point was that all of the various single number capability indexes that have shoved down our throats by the AIAG and others are fatally flawed and diversionary.

These organizations have tried to make a complex topic far too simple. Simple to the point where the true variation is hidden from view. We have abandoned our common sense.
 

Statistical Steven

Statistician
Staff member
Super Moderator
#13
Darius - first I don't think my sarcasm came thru in translation.
of less importance is the fact that just because customers ask for Cpk/Ppk doesn't mean they are right.

and I do believe that you can't manage without measuring but poor measurements only lead to poorer decisions. I've stated my distaste for trying to reduce the beautiful complexity of variation to a single number in previous posts. Manager's who can't interpret simple charts like run charts, multi-vari charts and control charts need to be taught as do our customers. WE need to be intellectual leaders, not lemmings who follow the priests of simplistic off the cliff.
Bev -

Actually, you are raising an age old issue with capability indices. I think 6S got a foothold in many companies because they converted from Cpk/Cp/Pp/Ppk world to a world of DPMO or PPM. The reality is that you really DO want to know what is the probability of making out of specification product regardless of where the closest specification may lie.
 

Bev D

Heretical Statistician
Staff member
Super Moderator
#14
Bev -

Actually, you are raising an age old issue with capability indices. I think 6S got a foothold in many companies because they converted from Cpk/Cp/Pp/Ppk world to a world of DPMO or PPM. The reality is that you really DO want to know what is the probability of making out of specification product regardless of where the closest specification may lie.
Yes I agree that we need to know how many defective parts we are making - which is why I never got the use of only the worst side of the process for the Ppk value. If we have defects to the low and high side then we should report it. Using only the worst side under reports the defect rate. (and dont' get me started on the Normal distribution problems)

The other stated reason for the capability indices is to provide data on spread vs tolerances in case:
  • there is a possibility of tolerance stackups
  • the tolerances have not been engineered
  • there is no true zero defect state only a minimized defect zone (Taguchi's loss approach idea)
But again, the index itself oversummarizes the data and censors one tail.
 
A

Allattar

#15
Bev, youve hit the nail on the head.
A Cpk on its own is surely only showing, possibly, half the story. It needs to be used with PPMs or DPMO figures to fairly represent the process.

If you are going to report just one number then surely the Zbench score is a better measure to use instead of Cpk scores.

Of course if I wanted to throw something else into the discussion how about the switch of Cpk, Ppk being refered to as short term or long term. Depending on who you talk to.

The other great confuser in all of this is the sigma shift issue. Which alone is enough to make anyone start to twitch. Had plenty of fun discussions on that, why are you adding/removing 1.5 sigma? what logic are you applying there?
 

bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#16
Caster - i think you too lost my sarcasm. My examples were intended to point out the weaknesses and fallacies of the capability index.

...My point was that all of the various single number capability indexes that have shoved down our throats by the AIAG and others are fatally flawed and diversionary.

...(and don't get me started on the Normal distribution problems)
Actually, I'll support you on the normal distribution problems. Precision machining is primarily non-normal (as has been discussed elsewhere in detail). Most customers are drooling over Cpk for these processes, yet AIAG itself states that Cpk and Ppk DO NOT APPLY TO NON-NORMAL DISTRIBUTIONS. Unfortunately, many folks skipped that chapter of PPAP 4th edition, and mess with the calculations anyway. Pity. In precision machining, the better the Cpk, the more evidence you have that you are overcontrolling the process. Feel better about those high Cpks? Would make a Lean practitioner sick.
 

Stijloor

Staff member
Super Moderator
#17
Actually, I'll support you on the normal distribution problems. Precision machining is primarily non-normal (as has been discussed elsewhere in detail). Most customers are drooling over Cpk for these processes, yet AIAG itself states that Cpk and Ppk DO NOT APPLY TO NON-NORMAL DISTRIBUTIONS. Unfortunately, many folks skipped that chapter of PPAP 4th edition, and mess with the calculations anyway. Pity. In precision machining, the better the Cpk, the more evidence you have that you are overcontrolling the process. Feel better about those high Cpks? Would make a Lean practitioner sick.
Why not getting out of the Process Capability indices game all together?
Most of these indices are highly suspect anyway.
Capability indices are supposed to be based on a stable process which is seldom the case.

Stijloor.
 

reynald

Quite Involved in Discussions
#19
Most of these indices are highly suspect anyway.
Capability indices are supposed to be based on a stable process which is seldom the case.

Stijloor.
For JMP users, i created a script to demonstrate Cp and Cpk.
But i must say that among many, i dont agree on using a single value to assess your process. Better yet, plot the histogram, and create a control chart. That draws the picture that are mostly needed.

>>rey
 

reynald

Quite Involved in Discussions
#20
I missed the attachements.

Edit: *.jsl files are not uploaded. I'll post it as a text file instead
 
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