Six Sigma in Plain Words

Every organisation makes mistakes. A warehouse sends the wrong item. An invoice has the wrong price. A pipe is cut a few millimetres too short. Each mistake costs money and patience, and customers remember it.

Six Sigma is a disciplined, data-based way to find why these mistakes happen and to make them rarer. The name comes from the Greek letter sigma (σ), which statisticians use for the standard deviation: a number that shows how much results spread around their average. Less spread means fewer results outside what the customer accepts.

The phrase "Six Sigma" is used in two ways:

• A goal. A process working "at six sigma" makes about 3.4 defects per million opportunities. Module 6 explains where that figure comes from and the assumption behind it.
• A method. A project-based way of working, usually in five phases called Define, Measure, Analyze, Improve and Control (DMAIC).

What Six Sigma asks of people

Six Sigma rests on a few simple habits:

• Start from the customer. A defect is anything the customer would not accept.
• Use facts, not opinions. Measure the problem before you try to fix it.
• Look for causes, not culprits. Most mistakes come from the process, not from careless people.
• Reduce variation. A process that is sometimes perfect and sometimes poor is not under control.
• Hold the gain. A fix that fades after three months is not a fix.