Statisticians use the Greek letter sigma to express standard deviation, which refers to the average difference between a given point in a set of data and the average of all other points. In a quality context, that might mean comparing measurements for manufactured screws. A quality process that meets a performance goal of one sigma will produce slightly more than two out of three screws that meet specifications. Reaching two sigma means more than 95 percent of all screws will pass muster. By the time you get to six sigma, only 3.4 defects would show up in a million screws.
The use of Six Sigma in quality management originated, or at least was popularized, at Motorola. About 15 years ago, Motorola was trying to get its thousands of cost-oriented, technically-savvy engineers more concerned about quality. The odd-sounding name was considered appropriate to these goals.
"Six Sigma was a good way to describe quality to engineers," according to James Morehouse, a quality consultant with A.T. Kearney Inc., a global management consulting company in Chicago. "It was a tool for changing a very entrenched, engineering-driven organization from focusing on cost to focusing on quality. That's what Six Sigma is really all about."