What is the conversion for PPM defects vs. Sigma Equivalents?

[Statistical Steven]

The 1.5 sigma shift is a theoretical abomination which assumes that operators measuring things in production will not react until hit over the head, or that reactions only occur after changes in the process are actually charted, neither of which is true. IMO, the shift is the prime problem with SS, and a sure indication that SS advocates don't understand what production workers do for a living.

I agree 100%. The notion of DPMO is meaningless if you build in an assumption of 1.5 sigma shift, when in reality, this might not be the case. I actually read a nice commentary on SS regarding the 1.5S shift. In summary, it stated that with measurement and process improvements, the 1.5S should be reduced to 1.0S with the intention of eliminating it over time.

 

[Barbara B]

Maybe they just choose a 1.5 shift because "Six Sigma" sounds much better than "Five Sigma" or "Four point Five Sigma"

Barbara

 

[e006823]

I usually don't respond to these 6 Sigma discussions but here goes. Having been involved with 6 Sigma in one form or another for about 10 years the following is my understanding of the "1.5 Sigma Shift" concept.

The "1.5 Sigma Shift" does not mean that a stable process will shift 1.5 sigma without being detected but that it could shift 1.5 sigma and since the process is so robust (e.g. 6 Sigma) this shift would be of no consequence to the customer.

Regards

 

[Jim Wynne]

I usually don't respond to these 6 Sigma discussions but here goes. Having been involved with 6 Sigma in one form or another for about 10 years the following is my understanding of the "1.5 Sigma Shift" concept.

The "1.5 Sigma Shift" does not mean that a stable process will shift 1.5 sigma without being detected but that it could shift 1.5 sigma and since the process is so robust (e.g. 6 Sigma) this shift would be of no consequence to the customer.

If there's no concern that the process will shift, then what's the point? My point was that if the process is operating normally, and comfortably within the spec limits but nowhere near six sigma, and things start to change, the change will almost always be detected before it gets to the point where it would be of consequence to the customer. It's called process control. And by the time it is detected (i.e., if a full 1.5 sigma shift happens before anyone notices anything went wrong) there are likely to be consequences anyway. The 1.5 sigma shift concept is theoretical BS because it assumes that operators are robotic dolts who wait for anomalies to actually be charted before responding.

 

[e006823]

The 1.5 sigma shift concept is theoretical BS because it assumes that operators are robotic dolts who wait for anomalies to actually be charted before responding.

The 1.5 sigma shift or Six Sigma for that matter assumes no such thing about operators. The operators are the process experts and are key players in the implementation of the program and any subsequent process improvement efforts


The following is an excerpt from Six Sigma - Understanding the Concept, Implications and Challenges, by Mario Perez-Wilson, who played a role in the promotion and implementation of Six Sigma at Motorola and is the best explanation of the origins and reason for the use of the 1.5 Sigma Shift I’ve seen:

“Six Sigma is not 3.4 ppm. The whole misunderstanding about 3.4 ppm resulted from Motorola’s document “Our Six Sigma Challenge”. In it Motorola asserted if a process was made to be Six Sigma by having the design specifications be twice the process-width, the process would be extremely robust. Such a process would be robust, even if it was surprised by a significant or detrimental shift in average, as high as +1.5 sigma, the customers would not perceive a degradation in quality. At worst case, a shift of 1.5 sigma, would make a zero-defects product be 3.45 ppm and the customer would only perceive an increase from zero to 3 products defective, assuming a production run of 1,000,000. This was supposed to be the warranty Six Sigma processes brought to the customer, not actual ppm levels for Six Sigma.”

Regards

 

[Jim Wynne]

“Six Sigma is not 3.4 ppm. The whole misunderstanding about 3.4 ppm resulted from Motorola’s document “Our Six Sigma Challenge”. In it Motorola asserted if a process was made to be Six Sigma by having the design specifications be twice the process-width, the process would be extremely robust. Such a process would be robust, even if it was surprised by a significant or detrimental shift in average, as high as +1.5 sigma, the customers would not perceive a degradation in quality. At worst case, a shift of 1.5 sigma, would make a zero-defects product be 3.45 ppm and the customer would only perceive an increase from zero to 3 products defective, assuming a production run of 1,000,000. This was supposed to be the warranty Six Sigma processes brought to the customer, not actual ppm levels for Six Sigma.”

I'm not confused about 3.4 ppm or anything else, so this whole passage is irrelevant. In fact, it only serves to support my contention, because it confirms the purpose of the 1.5 sigma shift mythology in line with my earlier post. It supposes that a significant shift would come as a "surprise." And answer me this: If the process is "robust" after such a surprise, why is it necessary to extend the limits to six sigma? Why not just use the "robust" limits and use process control to insure that it remains "robust"?

 

[Bev D]

the 1.5 sigma shift is an abomination, but since it really has NOTHING to do with six sigma (other than the six sigma = 3.4ppm tag line - as in Nike's "just do it" and Intel's 'Intel Inside') all good - and better - six sigma practitioners simply ignore it. It's not taught and it's not used. We may only use it to designate a process as beign 3 sigma or 5 sigma, but even that is falling away to describe a process by it's real defect performance level.

It's really unfortunate that the bizarre 1.5 sigam shift ever got tacked onto Motorola's Six Sigma approach - it's done nothing but cause diversionary, divisive and ultimately useless discussions that have simply detracted from the real core of Six Sigma...

 

[Jim Wynne]

the real core of Six Sigma...

Which, without the shift, is just good ol' fashioned process control taken to extremes and dressed up in an ill-fitting new suit.

 

[Statistical Steven]

Which, without the shift, is just good ol' fashioned process control taken to extremes and dressed up in an ill-fitting new suit.

I will throw in that is can also be called TQM....

I will go to my grave saying that six sigma is just TQM repackaged.

 

[Bev D]

and your point is? if it works who cares what you call it? I am seeing real statistical engineering quality improvements and control being enacted in numerous organizations under the name of six sigma that I never saw under process control or tqm...i'm just thrilled more people and organizations are actually using and embracing the techniques and systematically ingraining them into their organizations...I just dont get the visceral disdain around the term (seemingly bitter?)


I understand the disdain for the hacks (like Mikel Harry) who used good old american capitilism or snake oil salesmanship and made tons of money off the name gimmick - but these guys are always out there.