Origin and meaning of the term six sigma process :

The term six sigma process comes from the notion that if one has six standard deviations between the process mean and the nearest specification limit, as shown in the graph, practically no items will fail to meet specifications. This is based on the calculation method employed in process capability studies.

Capability studies measure the number of standard deviations between the process mean and the nearest specification limit in sigma units. As process standard deviation goes up, or the mean of the process moves away from the center of the tolerance, fewer standard deviations will fit between the mean and the nearest specification limit, decreasing the sigma number and increasing the likelihood of items outside specification.

Variations in a process and mean shift are two different things, as opposed to what seems suggested in the post. For example, let us take 10,20 and 30. What is the mean of these three = 20 right?

What is the variation, 66.6 yeah?

Good, now let us take this for example, 30,30,30. What is the mean here? 30 right. What is Standard Deviation == 0, right.

The mean has shifted here. Remember I have taken a sample here. If this process was to continue giving data with a mean of 30, it would still have variations, which means it would have a mathematical value of std dev.

Per definition of std dev, it is measured as std dev from mean, not the other way around.

Thanks
Vishwanathan
LSS MBB and Lean Deployment Specialist.