Six Sigma is a data-driven method for improving quality by reducing variation and defects in a process. Teams define the problem, measure it, find the root causes with statistics and control the result, usually through a five-phase cycle called DMAIC.
This guide explains what 6 sigma means in numbers, where the method came from, its core principles, how DMAIC works, the belt levels and how Six Sigma fits with lean. Each section links to a deeper page.
Key takeaways
- Six Sigma reduces variation, so a process gives the same good result every time.
- A six sigma process, by the usual convention, produces about 3.4 defects per million opportunities.
- Projects follow DMAIC: define, measure, analyze, improve and control.
- Belts, from white to master black belt, describe how much training and project experience a person has.
- Lean Six Sigma pairs lean's focus on flow and waste with Six Sigma's focus on variation.
What is Six Sigma?
Six Sigma is both a measure and a method. As a measure, it describes how often a process produces defects. As a method, it is a structured way of running improvement projects with data.
The core idea is that customers feel variation, not averages. A delivery that is on time on average but sometimes a week late still lets customers down. Six Sigma aims to make the process consistent, then keep it there.
Six Sigma projects are run by trained practitioners, with a sponsor from management and a clear financial or customer goal. They often use statistical tools such as hypothesis tests, regression and designed experiments, alongside simple ones such as the fishbone diagram.
What does 6 sigma mean in numbers?
Sigma (σ) is the Greek letter used for standard deviation, a measure of spread. A process at six sigma has its specification limits six standard deviations from the mean, so very little of its output falls outside them.
By convention, Six Sigma tables allow for the process mean to drift by 1.5 sigma over time. With that shift, six sigma equals about 3.4 defects per million opportunities (DPMO). Four sigma is about 6,210 DPMO and three sigma about 66,807.
You can turn your own defect counts into a sigma level with the Six Sigma calculator, and check how a measured process fits its limits with the process capability calculator.
Where Six Sigma came from
Six Sigma was developed at Motorola in the mid-1980s. Engineer Bill Smith is usually credited with the approach, and Motorola registered Six Sigma as a trademark.
General Electric adopted it across the company in the mid-1990s under chief executive Jack Welch, which made it widely known. It then spread to other manufacturers, banks, hospitals and service companies.
Its statistical roots are older. Walter Shewhart's control charts from the 1920s and W. Edwards Deming's teaching on variation are the base that Six Sigma builds on. See statistical process control.
Six Sigma principles
Most descriptions of Six Sigma come back to a handful of principles.
Focus on the customer: define quality by what is critical to the customer, often written as CTQs (critical to quality requirements).
Use data: measure the process and verify causes with data before acting, rather than relying on opinion.
Reduce variation: work on the spread of results, not only the average, because customers feel the bad days.
Improve the process: treat defects as products of the process, and fix the process rather than blaming people.
Sustain the gain: build in controls, such as control charts and standard work, so results do not slide back.
DMAIC: the Six Sigma cycle
Define sets the problem, scope, goal and customer requirements in a project charter.
Measure checks the measurement system and builds a baseline of how the process performs today.
Analyze finds and verifies the root causes of defects and variation.
Improve tests and implements solutions that remove those causes.
Control keeps the gain with a control plan, monitoring and updated standards. The DMAIC page covers each phase, its tools and the tollgate reviews in more detail. New processes use a variant called DMADV: define, measure, analyze, design and verify.
Six Sigma belts
White belt
A short introduction to Six Sigma terms and roles, for anyone working near a project.
Yellow belt
The basics of DMAIC and simple tools, so a person can take part in projects as a team member.
Green belt
Enough statistics and project skills to lead smaller projects, often alongside a regular job.
Black belt
Advanced statistics and change leadership, to lead larger projects full time and coach green belts.
Master black belt
An expert who trains and mentors black belts and helps choose projects for the organization.
Six Sigma certification
There is no single official body for Six Sigma. Certifications come from professional associations, universities, consultancies and employers, and they differ in exam rigor, project requirements and cost.
Before choosing, check whether the program needs a completed project, whether the exam is proctored, and whether employers in your field recognize it. The Lean Six Sigma certification guide compares providers.
What is Lean Six Sigma?
Lean Six Sigma combines the two approaches. Lean removes waste and speeds up flow; Six Sigma reduces variation and defects. Together they address both how fast and how reliably a process delivers.
In practice, Lean Six Sigma projects follow DMAIC and use lean tools such as value stream mapping, 5S and kanban alongside statistical ones. The lean vs Six Sigma guide explains where each fits.
Six Sigma tools and software
Common Six Sigma tools include SIPOC diagrams, process maps, Pareto charts, fishbone diagrams, measurement system analysis, capability studies, hypothesis tests, designed experiments and control charts.
Statistical packages and project tracking tools support this work. The Six Sigma software and SPC software buying guides list options, and the Lean Six Sigma tools hub covers the rest of the stack.