Is 2 Sigma 69.20% or 95%? I Am Confused!

There are two aspects to the conversion of the percent defect free product to its corresponding sigma level- one is the mathematical aspect and the other is the operational aspect.
The mathematical aspect may be understood through a function in MS Excel which is normsinv(probability of defect free product). When using this function, you have to enter the probability of making a defect free product as the parameter for this function. The result from this function gives the mathematical way of converting the probabluty into the sigma level. Using this function, you would find that the sigma level corresponding to 99.9997% defect free product is 4.5 sigma. Entering 69.2% or 0.692 is the parameter, we would find that the corresponding sigma level is 0.5
If we wish to use the concept of 1.5 sigma shift, which takes into account the inherent deterioration of the systems and processes involved in generating the product or service, we would have to add 1.5 to the above function which I have told. Adding 1.5 sigma to the result obtained, we would find that 99.9997% or 0.999997 corresponds to 6 sigma (which is the widely accepted defintion of six sigma). The sigma level corresponding to 0.692 would then be 2 sigma.
In case you have more doubts, please feel free to get in touch with me directly on my mail ID [email protected] or call me 91-9818550488
Regards
Sudeep Bhargavahttp://www.globaladroit.com

Every real system in this world deteriorates with time. There is nocontrol over this deterioration, It is true with the human body as well as the other physical systems including the business processes. 1.5 sigma shift takes into account that “deterioration” only.
I am sure Mr Stan would like to enlighten all the users here with his knowledge of this “1.5 sigma shift”.
Meanwhile, the readers may refer to the following link from isxisigma.com
https://www.isixsigma.com/ library/ content/ c010701a.asp
Thanks in advance
Sudeep Bhargava

I don’t understand why six sigma folks bother about short term sigma at all. Doesn’t the customer actually experience the long term and not the short term? Why bother to compensate for shifts? Just report it as it is.

Shhhhhh!It’s more fun to set these fools up.

Sushil,I am willing to participate in healthy debate.What is your take on the subject and why?

Would you know the correct answer if you heard it. And how do
you know?This is what I would ask any interviewer dumb enough to think
this is a good question. And yes, I know the answer.

You are suffering under the worst and most confusing topic in the six sigma world.  Some tables show sigma levels with the 1.5 sigma shift baked in, others show raw sigma levels.  Is 1.5 the “right” correction between short and long term, and for that matter, what is “short,” and what is “long?” 
If you are properly monitoring your process, you should be monitoring the critical factors which cause your important results to change, and as they change from the acceptable level of variation (due to deterioration from wear or other uncontrolled forces), they are replaced or otherwise brought back within the acceptable range before your critical output varies beyond acceptance.  Thus, you will not observe a 1.5 sigma shift.
You may find a mathematical derivation as to a 1.5 sigma shift (search for Davis Bothe), but most fall back on experiential data. 
Monitor your inputs and your outputs will not suffer from the “shift.”
 

If you think that you know “the answer” then I wouldn’t want to work for you.

Dear Stan,
Thanks,
At my place we go with the short term sigma without taking that 1.5 into account.
Reason: Most of the times due to nature of operations (automated measures and miniscule human intervention in meassurement where R&R is always less than 2 % or so) we do so.
But this +1.5 sigma takes care of lot many factors which we may not consider in short term and external parties like customer always experience the gap in our processes.
In  my opinion, we measure to improve and if we see that ST Sigma is fulfilling my purpose and I have enough scope to improve,  will go for ST other wise explore that unseen  factors by using LT Sigma. Totally depends uppon the process I am dealing with and it’s importance for me.
In my opinion it’s all about improvement with right measurement.
Thanks

Stan knows but he would have to shoot you if he told you. There is no such thing as “a shift” therefore no calculation. Agreed that there is some inherent drift of a process over time. A simple control chart shows the amount of drift. In some cases it is not significant, in others it is. Doing a process capability with both short term and long term measures of variation indicate to what degree the shift is something to worry about. If the Cpk is essentially the same as the Ppk then who cares about the drift. If there is significant difference then you worry. To apply a fixed number is foolish. Even Harry called his shift a “fudge factor”. Great for Forum discussions but pretty useless for business purposes.