# 1.5 Sigma Process Shifts

Six Sigma – iSixSigma Forums Old Forums General 1.5 Sigma Process Shifts

This topic contains 12 replies, has 7 voices, and was last updated by  Grant Blair 18 years, 5 months ago.

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• #27088

Joe Perito
Participant

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#65886

Joe Perito
Participant

Here’s a correction to my own posting on 1.5 sigma shifts: All of the logic is OK in the posting but as soon as I signed off I realized I was not paying enough attention to the math. For beginners in the Quality field be advised that a CpK of 1.33 is a plus or minus 4 sigma process. A process potential study that yields a PpK of 1.67 is a +/- 5 sigma process. Both of these calculations are based on a 3 sigma process yielding 99.73% good quality. The 3 is always in the denominator of the two formulae. Therefore, to get a CpK of 1.33 you divide a (+/-) 4 sigma process by 3 to get a CpK of 1.33. A PpK of 1.67 is a +/- 5 sigma process divided by 3 (5/3 = 1.67).

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#65919

Kim Niles
Participant

Thanks for your posting. This is a very interesting subject to me.

I see what you are saying but take on the argument that since “Six Sigma” is more of a management system than a statistical metric, any excuse one could find to keep seemingly unrealistic management goals from being obtainable is ok by me.

Besides that, this subject will always be controversial as it was before Six Sigma came along. Articles date back to 1975 that I know of on whether or not a shift should be applied to process metrics.

KN

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#66002

Cone
Participant

There is no debate as to whether processes shift or not. Those who wonder about such things only need to analyze some long term data from any process.

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#66042

Kim Niles
Participant

Dear Gary:

I agree that processes shift.

I believe that the 1.5 sigma shift controversy is all over whether or not processes that are “stable” shift or not. The controversy lies in the definition of one word (stable).

If we have a perfectly stable process that includes all the variation that the process will ever see in the standard deviation calculations then adding a 1.5 sigma shift means that we have a 4.5 sigma process at 6 sigma.

Right?!?!??!

KN

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#66135

Charlie Pfaff
Participant

I may be getting lost with six sigma, but I thought the “new” cpk is done over a short term part of the process, while a ppk is done over a long term part of the process where all the variables of the process pertake in the data. A short term is where a minimum amount of the variables effect the data. Is this correct. A Ppk of 1.67 would probably be better than a Cpk of 1.33, but according to Motorola a cpk could undergo a shift of 1.5 standard deviations. I do not agree, it depends on the process. In parts manufacturing alignment and operator influence would all be in the short term. Maintenance, possibily tool life and coolant changes would not. If tool life was in the short term, I doubt if there would be that much of a shift and maintenancer variability also usually shows up in affecting the standard deviation not the X bar shift. “Short term and Long term” need to be explained to a more specific degree. If I run my process and all the variables of the process have a chance to affect my data; if I have a Ppk (old Cpk) of 1.67 my process is in great shape.

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#66136

Ken Myers
Participant

Charlie,

You are correct in your thinking about Cpk and Ppk. There is no change here. Not sure what is meant by the “new” Cpk, but the your thinking appears reasonable to me. Cpk is a measure of process capability using short term variation, and Ppk is the same measure using long term variation. Certainly, a Ppk of 1.67 would be better than a Cpk of 1.33.

Much discussion has come and gone concerning Motorola’s +/-1.5 sigma shift. We have all covered this subject extensively in the last week or two, and most are probably exhausted talking about it further. If you view the message board closer I believe you will find your thoughts have generally been conveyed.

As Gary Cone and others have indicated if you do not have any long term data supporting the process, then the +/-1.5 sigma shift advocated by Motorola can be used as a conservative estimate. As you collect the additional data necessary to compute the long term variation a better estimate of the process shift can be made. Hopefully, I’ve done a reasonable job of conveying our collective understanding here. There is considerable controversy and emotion surrounding this topic, but I believe most would agree with my comments above.

Good Luck,

Ken

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#66137

Ken Myers
Participant

Kim,

Yes, you are correct…

Ken

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#66142

Cone
Participant

Amen. If you really have Ppk’s of 1.67 for all your critical processes, you are in great shape. If fact, if you have such a situation, I would pay my own expenses to see it. You would be better than any of the folks implementing Six Sigma

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#66150

Cone
Participant

Charlie,

You are right, but I want to make a couple of points on what you wrote.

“I may be getting lost with six sigma, but I thought the “new” cpk is done over a short term part of the process, while a ppk is done over a long term part of the process where all the variables of the process pertake in the data.”

While this is technically correct, I would suggest that “new” Cpk is meaningless except where you have a one sided spec, off center target, or some type of process where the mean cannot be economically adjusted to center. In all other cases Cp is important, not Cpk.

“A short term is where a minimum amount of the variables effect the data. Is this correct?”

This is correct.

“A Ppk of 1.67 would probably be better than a Cpk of 1.33”

Orders of magnitude better.

“but according to Motorola a cpk could undergo a shift of 1.5 standard deviations. I do not agree, it depends on the process.”

Absolutely, only assume 1.5 when you do not have real data to tell you what to expect.

“In parts manufacturing alignment and operator influence would all be in the short term.”

This is only true where all management and operators are trained and guaranteed to be consistent prior to being left alone to do their jobs. If not, this is a huge source of long term variability.

“Maintenance, possibily tool life and coolant changes would not. If tool life was in the short term, I doubt if there would be that much of a shift and maintenancer variability also usually shows up in affecting the standard deviation not the X bar shift. “Short term and Long term” need to be explained to a more specific degree. If I run my process and all the variables of the process have a chance to affect my data; if I have a Ppk (old Cpk) of 1.67 my process is in great shape.”

Absolutely true. Short term is just snapshots of your process. Long term is everything. The 1.5 Motorola suggests is usable for two things. It is a rational assumption if you do not have real data, but have basic controls in place including a guarantee of management and worker training and alignment. It is a gross underestimation if the guarantee is not in place. God bless you and your organization if you can have more than a theoretical conversation about all of your process being at a Ppk of 1.67. Your organization is rare and in good company only with Toyota as far as I know.

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#66159

Grant Blair
Participant

This is my first try at a post on this discussion group.
Stop thinking about how much a process MIGHT shift.
ALL processes shift…it’s a law of nature called
entrophy which all processes MUST follow (Otherwise,
you could build perpetual motion machines )
Now, given that a process WILL shift, what is the
maximum shift which COULD occur before you detect it
and correct the process to keep from making defectives?
If you want to only protect PPthousand level, allow 1 sigma,but if you want to protect PPmillion, allow 1.5 sigma as protection.

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#66170

Anonymous
Participant

Say what?

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#66212

Grant Blair
Participant

If I control chart a Cpk=1.5 process with a 0 target and a sigma of 1 and the X-bar (or X)chart indicates a point beyond 3 sigma, then how much should the process be adjusted to bring it back to target?
1.What would happen to defect levels if you ALWAYS adjusted 3 units?
2. What would happen if you ALWAYS adjusted 1 unit?
3.What would happen if you ALWAYS adjusted 1.5 units?
If you also include trend/runs rules to detect special casue, then how should the process be adjusted?
1.What would happen if you ALWAYS adjusted by the distance of the last point to the target?
2.What would happen if you ALWAYS adjusted by 1/2 the
distance of the last point to target?
Would any of the earlier rules be better than the last two?

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