Six

Six sigma is very famous concept invented in GE.

In this concept, the business process is elaborated aiming at the higher quality; sigma is the variance in statistics, and six sigma is that the quality is as high as the probability that the sample included to the rejection region, μ±6σ, is 3.4/1,000,000.

But actually, assuming the normalized distribution, the probability above is 2/1,000,000,000. This is because of the basic assumption for six sigma is the idea related the User Specification Limit, the concept of industry.