Six Sigma and the Mysterious “1.5-Sigma Shift”


As I mentioned in my previous post, sometimes the most interesting discussions in our Six Sigma Green Belt class occurnot in class, but before the class has started when everybody has not yet arrived and those who have arrived ask the instructor questions on what they have read over the previous week.

A couple of weeks ago, I came across something called the “1.5-Sigma Shift,” a mysterious phenomenon whereby companies are only achieving 4.5 Sigma on the average when they are trying to achieve Six Sigma. I asked our instructor Bob Mehta about this, and here’s his reply.

1. Theory one: Long-term “drift”

The one theory is that a Six Sigma project may achieve a short-term quality improvement, but then this fades over time so that the system drifts back towards its previous state with poorer quality. The reason for this could that long-term habits of equipment operators, etc., may cause the system to revert to its earlier state even if there is short-term improvement.

2. Theory two: Root cause not identified

The second theory is that a Six Sigma project may achieve a short-term quality improvement, but then this is overshadowed by other forces which cause poor quality. The reason for this is not because of an equipment operator’s error, but rather that the design improvement caught one of the causes of the quality problem, but not necessarily all of them, and most importantly, not the root cause of them. So one “symptom” has been cured, but not the underlying “disease”, to put it in medical terms.

In our class discussion, we came to the conclusion that quality engineers might tend to pick theory one as the reason for the 1.5-Sigma drift because it puts less of the blame on them as compared to theory two.  At any case, it does show the importance of not taking any quality improvements achieved in the past for granted, but reviewing and revisiting them from time to time.

It was great to have such a candid discussion with our instructor.

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