sigma represents standard deviation. In a normal distribution, +/- 1 sigma from the mean, for instance, corresponds to approximately 67% of the area under the normal distribut

…ion. +/- 2 sigma corrresponds to 95% of the area and +/- 3 sigma from the mean corresponds to 99% of the area under a normal distribution. The area that is covered under +/- six sigma from the mean corresponds to nearly 100% -- that is, the part of the area NOT under that +/- 6 sigma is in the 10^-15 range or 1/1,000,000,000,000,000. The six sigma name borrows from this to suggest that the method gives this degree of certainty: that in 999,999,999,999,999 in 1,000,000,000,000,000 cases the result will be predictable. It does nothing, however, to explain how an agricultural process management methodology applies to other fields. ( Full Answer )