6 Sigma
Let's start with two words. A "standard deviation" (written with the Greek letter sigma, Ο) is a number that tells you how spread out your data is β how far, on average, individual results wander away from the average result. The "mean" is simply the average. When we talk about "6 Sigma," we mean the range that stretches six standard deviations above the average and six below it.
The formula 6Ο = 99.99966% tells you that if a process is running at a Six Sigma level of quality, then 99.99966% of everything it produces falls within that very wide, healthy range β and only about 3.4 items per million come out defective. A "defect" just means a result that doesn't meet the requirement.
How do you read this? The bigger the sigma level, the better the quality. A process at 6 sigma is extraordinarily reliable β almost nothing goes wrong. A lower sigma level (like 3 sigma) allows far more defects. Project managers use this idea when quality really matters, such as in manufacturing, healthcare, or aviation, where even tiny error rates are unacceptable.
Think of it like aiming arrows at a target. A wobbly beginner (low sigma) scatters arrows all over the wall. A world-class archer (six sigma) puts virtually every single arrow in the bullseye β only about 3 arrows in a million ever miss the mark.
Imagine a factory that fills 1,000,000 bottles of medicine. If this process runs at a Six Sigma level of quality, you would expect only about 3.4 bottles out of that entire million to be filled incorrectly. That means 999,996.6 bottles are perfect β which is why 6Ο is described as 99.99966%. If a hospital demanded this level of quality, they'd be trusting that almost no dose ever goes wrong.
Every PMP formula explained free β plus worked examples and practice in PMP Math, and full timed mocks in the simulator.