Six Sigma vs. Lean Six Sigma: What Is the Difference and Which Should You Learn?
Six Sigma attacks variation: results that swing from one run to the next. Lean attacks waste: steps, waits and work that add no value. Lean Six Sigma combines both. This guide explains each approach, compares them side by side, walks through a worked example, and helps you decide which to study first.
The short answer
- Six Sigma asks: “Why do results vary, and how do we make them consistent?”
- Lean asks: “Which steps add no value, and how do we remove them?”
- Lean Six Sigma uses both sets of questions on the same process.
ASQ, the professional body for quality, puts it in one line: lean focuses on waste reduction, whereas Six Sigma emphasizes variation reduction. ASQ also notes that the distinction has blurred, and that the combined term “Lean Six Sigma” is used more as a result.
Most beginners today meet them together. Many training providers teach one combined curriculum.
What Six Sigma is
Six Sigma is a method for disciplined quality improvement. It uses data and statistics to find the causes of defects and to make a process more stable.
Key features:
- A target: a process at “six sigma” quality is expected to produce no more than 3.4 defects per million opportunities. That figure assumes a 1.5-sigma shift in the process over time.
- A method: projects follow DMAIC (Define, Measure, Analyze, Improve, Control) to fix an existing process, or DMADV to design a new one.
- Statistical tools: process capability, hypothesis tests, designed experiments and control charts.
- Roles: Champions, Master Black Belts, Black Belts and Green Belts, covered in our belt rankings guide.
For the basics, read what Six Sigma is.
What lean is
ASQ defines lean as “a set of management practices to improve efficiency and effectiveness by eliminating waste.” Lean looks at how work flows and removes anything that does not create value for the customer.
A common list names eight kinds of waste. The memory aid is DOWNTIME:
| Letter | Waste | Office example | Factory example |
|---|---|---|---|
| D | Defects | An invoice sent with the wrong amount. | A part that fails inspection. |
| O | Overproduction | A report nobody reads. | Making more than customers ordered. |
| W | Waiting | A request sits in an approval queue. | A machine idle until material arrives. |
| N | Non-utilized talent | Skilled staff doing data entry. | Operators never asked for ideas. |
| T | Transportation | Files passed through many hands. | Parts moved long distances between steps. |
| I | Inventory | A backlog of unprocessed forms. | Piles of work-in-progress. |
| M | Motion | Searching for a document. | Walking to fetch tools. |
| E | Extra processing | Three approvals where one would do. | Polishing beyond what the customer needs. |
The examples are ours. Some authors list seven wastes and leave out non-utilized talent. ASQ discusses the seven-versus-eight difference on its lean page.
Typical lean tools include workplace organization, visual controls and kaizen events, which are short, focused improvement efforts.
What Lean Six Sigma adds
Lean Six Sigma is not a third, separate method. It is a toolkit that applies lean thinking and Six Sigma statistics to the same project, usually inside the DMAIC structure.
- Lean finds where time and effort are wasted.
- Six Sigma measures how much variation and how many defects remain.
- DMAIC gives the team a path from problem to lasting fix.
The combination makes sense because many real problems have both causes. A slow process is often also an inconsistent one.
Side-by-side comparison
| Feature | Six Sigma | Lean | Lean Six Sigma |
|---|---|---|---|
| Main target | Variation and defects. | Waste and flow. | Both. |
| Core question | Why are results inconsistent? | What adds no value? | What is slow, wasteful and inconsistent? |
| Typical measure | Defects per million opportunities, process capability. | Lead time, wait time, work in progress. | All of these. |
| Typical tools | Statistical analysis, designed experiments, control charts. | Process maps, visual controls, kaizen events. | A blend, organized by DMAIC. |
| Math needed | Moderate to high. | Low. | Moderate, rising with belt level. |
| Speed of first results | Slower; data collection comes first. | Often faster for obvious waste. | Depends on the problem. |
| Best for | Hard-to-find causes of defects. | Delays, queues and clutter. | Most business problems. |
These are general patterns, not rules. A good practitioner uses whatever tool fits the problem.
A worked example
This is a made-up company, but the pattern is common. A billing team sends about 1,000 invoices a month. Customers complain that invoices arrive late and that some are wrong.
| Approach | What the team finds | What it does |
|---|---|---|
| Lean view | An invoice takes 9 days from job completion to sending. Only about 40 minutes is actual work; the rest is waiting for approvals and batch runs. | Removes one approval, sends invoices daily instead of weekly. |
| Six Sigma view | Around 8 percent of invoices have errors. Data shows most errors come from one input form with a free-text price field. | Changes the form to a pick list and adds a check, then tracks errors on a control chart. |
| Lean Six Sigma | Both problems exist, and they feed each other: errors cause returns, which add waiting. | Runs one DMAIC project that fixes the waits and the errors, then monitors both. |
The numbers are an illustration, not real data. The point is that either method alone solves half of the problem.
For the full project path, read Six Sigma DMAIC. For process basics, see what a business process is.
Where each one came from
| Approach | Origin |
|---|---|
| Lean | Grew out of the Toyota Production System, developed after World War II. The word “lean” was coined by John Krafcik in a 1988 article, and was spread by the 1990 book The Machine That Changed the World by Womack, Jones and Roos. |
| Six Sigma | Introduced at Motorola in 1986 by engineer Bill Smith. It was later adopted by companies such as Allied Signal and General Electric. |
| Lean Six Sigma | Combined by practitioners in the early 2000s, when books began using the joint name. |
For an earlier effort to study work scientifically, see scientific management.
Which one should you learn?
| If you are… | Start with | Why |
|---|---|---|
| New to improvement work | Lean Six Sigma Yellow or Green Belt | One course covers both toolsets. |
| In operations, admin or service | Lean Six Sigma | Delay and rework problems need both. |
| An engineer or data-minded analyst | Six Sigma statistics, then lean | You will use the statistical depth. |
| A manager who will sponsor projects | Lean basics, Green Belt overview | You need the language, not every formula. |
| Short on time | Lean basics | Lean ideas apply the next day. |
If you are unsure, choose the combined course. It is a widely offered format and covers both toolsets.
Belts and certification
Both Six Sigma and Lean Six Sigma use the same belt colors: Yellow, Green, Black and Master Black Belt. Higher belts mean more statistics and more project leadership.
- Compare options in our Lean Six Sigma certificate guide.
- See Six Sigma Green Belt for the usual first project-leader level.
- Read Six Sigma certification for how bodies and providers differ.
Certification is not regulated by one authority. Several bodies and many providers issue belts, and requirements differ. Check what an employer accepts before you pay.
Online providers offer introductory courses on Coursera and Udemy. We may earn a commission if you sign up, at no cost to you. We are not affiliated with any university.
Common mistakes
- Treating them as rivals. They solve different halves of the same problem.
- Using statistics too early. If a step is plainly wasteful, remove it first.
- Cutting too hard. Waste removal can strip out slack that protects quality.
- Collecting data nobody uses. Measure only what the project needs.
- Training without projects. Skills fade unless you apply them to a real problem soon.
- Ignoring people. Changes stick when the team understands and supports them.
Frequently asked questions
What is the main difference between Six Sigma and Lean Six Sigma?
Six Sigma targets variation and defects with statistics. Lean Six Sigma adds lean’s focus on waste and flow, and uses both on the same project.
Is Lean Six Sigma better than Six Sigma?
Not always. Lean Six Sigma suits most business problems because delays and defects usually occur together. A pure statistical problem, such as a hard-to-find cause of defects, may need deeper Six Sigma analysis.
What is the difference between lean and Six Sigma?
Lean focuses on waste reduction. Six Sigma emphasizes variation reduction.
Where did Six Sigma start?
At Motorola in 1986, introduced by engineer Bill Smith.
Where did lean start?
Lean grew from the Toyota Production System. The term “lean” came into use in 1988.
Do I need Six Sigma before Lean Six Sigma?
No. Most Lean Six Sigma courses start from the beginning and teach the needed Six Sigma tools along the way.
Which is easier to learn?
Lean concepts are usually easier to start with because they need less math. Six Sigma becomes harder at Black Belt level, where statistics play a larger role.
Browse certificate, course and degree programs side by side.