Six Sigma in one sentence
Six Sigma is a methodology for reducing process variation so that defects become statistically rare. The name comes from the statistical goal: if your process fits within six standard deviations of the mean, you produce 3.4 defects per million opportunities. That is an extremely tight window, and reaching it takes disciplined measurement.
Sigma levels and what they mean
Most manufacturing processes operate between 3 and 4 sigma, which translates to roughly 6,200 to 66,800 defects per million. At 5 sigma you are down to 233 defects per million. At 6 sigma, 3.4.
The practical takeaway: each sigma level represents an order-of-magnitude improvement. Moving from 3 sigma to 4 sigma does not require twice the effort. It requires a fundamentally different approach to how you measure and control your process.
Statistical process control
SPC (Statistical Process Control) is the engine behind Six Sigma on the factory floor. You collect data from your process, plot it on control charts, and watch for patterns that signal drift or instability.
A process can be "in control" (only common-cause variation) or "out of control" (special-cause variation present). The distinction matters because each type requires a different response. Chasing common-cause variation with quick fixes makes things worse, not better. Ignoring special-cause variation lets defects accumulate.
Roles in Six Sigma
Six Sigma uses a belt system borrowed from martial arts. Green Belts lead smaller projects within their work area. Black Belts lead cross-functional projects and mentor Green Belts. Master Black Belts set strategy and train others.
In practice, the titles matter less than the discipline. A factory with two people who genuinely understand control charts will outperform one with ten certified belts who treat certification as a resume line.
Where to start
Pick one process with a measurable quality problem. Collect data for two weeks. Plot a control chart. You will learn more from that exercise than from any textbook chapter. Six Sigma is not a philosophy to adopt wholesale. It is a set of tools you pull out when variation is costing you money, and you need to find out why.