I’ve been reading about the 1.5 sigma shift for some time. Several different interpretations have been published and people are losing sleep over this matter. Some have even driven a stake into the ground to fervidly defend their position. My intent in writing this column is to simplify understanding of 1.5 sigma shift in contributing to a greater ease of use for the Six Sigma methodology. Following are the best two definitions I could find on 1.5 sigma shift:

"1.5-sigma shifts and drifts: The theory that over time any process in control will shift from its target by a value of up to 1.5 sigma. Allowing for the 1.5 sigma shift results in the generally accepted six sigma value of 3.4 defects per million opportunities. Ignoring the 1.5 sigma shift results in a six sigma value of 2 defects per billion opportunities." (from ASQ’s Six Sigma Forum Magazine)

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