Great question. My question is why so many people never ask! It would drive me crazy until I knew! Anyway… A sigma is referring to the statistical concept of a standard deviation. The standard deviation is usually represented by the Greek letter sigma. The standard deviation tells us now much variation is in a process. For example, if you made baseballs, and if you could weigh them accurately enough, you would see that each one weighs a little different. Hopefully they are all within the specifications set forth by MLB, or whoever.
When products are very consistent with very little variation, they don’t cause much trouble. If we made baseballs and they had a lot of variation in their weight, it would be a problem, especially for the pitcher and probably the batter too! So low variation is good. So small standard deviations are good.
Because of some statistical properties of groups of data (“distributions”), we can count how many standard deviations can fit between the average and the upper specification of lower specification. The more the better. If we can only fit 3, we have a 3 sigma process, and are going to make a lot of defects. As the sigma level gets higher, it means we are getting more consistent, and making very few defects. When we get to 6 sigma, we only have about 3 to 4 defect out of every million things we make. That’s a very high quality level. Often times it also means less scrap and rework at the factory. So, many times we can make the quality better, and save costs at the same time. Pretty cool, isn’t it?
So why not go higher than 6? At times we do, like in airline safety, nuclear reactors, etc. But for many products, to get much past 6 sigma, you have to start spending more money, so the cool thing about saving money doesn’t work so well. And most companies find 6 sigma to be a tough enough goal by itself!
Six Sigma has also become a program, a workflow, a toolset, and may have other meanings to various organizations. In general, it is a methodology to help companies do their stuff Better, Faster, Safer, Lower Cost, and less Environmental Impact. That’s why so many companies are into it.