I am still puzzled by the 1.5 sigma shift.
Are you saying that the field survey, field data, field statistics are all different from internal survey,internal data and internal statistics? And that difference is about 1.5 sigma shift. Are there reasons for this shift? Is this the reason that process improvement has to base on customer input, i.e. the receiver of the output of a process so the 1.5 sigma shift can be covered?
The 1.5 Sigma shift is entirely arbitrary, yet the industry standard. Would a 1 sigma shift be more realistic? Perhaps, but it is not what Motorola used. Their field data indicated that even for processes they felt were controlled and capable, they experienced field failures characteristic of processes which had undergone a 1.5 sigma shift over time (in the field). Think of the 1.5 Sigma shift as a fudge factor, or safety factor. If we are imprecise in our estimates, we at least have some margin for error.
This really only comes into play when you are trying to estimate a Sigma Level based on field data. In this case, it does not really matter whether your process will actually fit this model: industry expectations of Sigma Levels include the 1.5 sigma shift.
Honestly, I think a Six Sigma deployment that focuses on Sigma Levels is a poor program. It should focus on money: increasing revenue and reducing costs. There is too much monkeying around that can be done to DPMO and Sigma Level to make them your focus. Money, on the other hand, is real and absolute.
As for customer input, the best reason to include it is to ensure you are impacting the issues that customers care about. I have heard Polaroid had some of the best efficiencies in the industry. Yet, they went bankrupt. A focus on internal efficiencies is not a guarantee of success, as you might just be the best organization in the world at filling your finished goods inventory with useless product. In the same way, it is easy to cut costs, but if you strip away your ability to provide the service your customers expect, you risk losing those customers and reducing revenue. So we can not just cut costs, we need to cut the non-value added costs. And who defines value? The customer. They are paying the bills.
Yet another example of how Six Sigma and Lean Six Sigma are the same thing!!
Learn more about the Lean Six Sigma principles and tools for process excellence in Six Sigma Demystified (2011, McGraw-Hill) by Paul Keller, in his online Lean Six Sigma DMAIC short course ($249), or his online Green Belt certification course ($499).