Lean vs Six Sigma: differences, history and when to use each

Lean and Six Sigma are both improvement methods, and many companies run them together. They start from different questions, though, and use different tools.

Lean asks where time and effort are wasted. Six Sigma asks why results vary. This guide compares the two and explains when to use each.

Key takeaways

Lean targets waste and flow. Six Sigma targets variation and defects.

Lean comes from the Toyota Production System. Six Sigma was developed at Motorola in the 1980s.

Lean Six Sigma combines both, often using the DMAIC cycle with lean tools inside it.

Use lean first when work waits and piles up. Use Six Sigma first when output is unpredictable or defects are hard to explain.

What is lean?

Lean is a way of improving work by removing waste, so value flows to the customer without delay. It grew from the Toyota Production System.

Its tools are mostly visual and practical: value stream mapping, 5S, kanban, SMED and standard work. The lean methodology guide covers the principles.

What is Six Sigma?

Six Sigma is a data-driven method for reducing variation and defects. Motorola developed it in 1986, building on work by Bill Smith and Bob Galvin.

The name comes from statistics. A process running at six sigma, allowing for a 1.5 sigma shift, produces about 3.4 defects per million opportunities. The sigma level calculator converts your own defect counts.

Six Sigma projects follow DMAIC: define, measure, analyze, improve and control. Practitioners are trained to levels named after martial-arts belts, such as Green Belt and Black Belt.

The Six Sigma guide covers the method, the belts and the statistics in more depth.

A short history of lean and Six Sigma

Lean grew out of the Toyota Production System, built by Taiichi Ohno and others from the late 1940s onwards. The Toyota Production System page covers its two pillars, just-in-time and jidoka.

The word lean came later. John Krafcik used it in a 1988 article. Womack, Jones and Roos made it widely known in their 1990 book The Machine That Changed the World.

Six Sigma began at Motorola in 1986. General Electric adopted it in the mid-1990s under Jack Welch, which spread it across large companies.

The two streams met in the late 1990s and 2000s. Consultants and companies combined them as Lean Six Sigma, often with DMAIC as the common frame.

Lean vs Six Sigma in manufacturing

In manufacturing, the difference between lean and Six Sigma shows up in the problems each one picks. Lean looks at the whole flow of material, from the supplier to the customer.

A lean team might find that a part waits days between two processes that each take minutes. The fix is flow: smaller batches, kanban pull and quicker changeovers with SMED.

A Six Sigma team looks inside one process. It might find that a machining step drifts out of tolerance on some shifts. The fix comes from data: measurement checks, a designed experiment and statistical process control.

Both meet on the shop floor. A stable, flowing line makes variation easier to see, and a capable process makes flow easier to keep.

Lean vs Six Sigma at a glance

Focus: lean looks at waste, waiting and flow. Six Sigma looks at variation, defects and capability.

Typical question: lean asks why an order takes days when the work itself takes hours. Six Sigma asks why some batches fail a test and others pass.

Tools: lean uses maps, visual boards and simple experiments. Six Sigma uses measurement systems analysis, hypothesis tests, regression and statistical process control.

Speed: lean improvements are often quick and visible. Six Sigma projects usually take longer, because they need data collection and analysis.

Skills: lean asks everyone to see and remove waste. Six Sigma relies on trained specialists for the statistical work.

What is Lean Six Sigma?

Lean Six Sigma combines the two. Lean makes the process flow, and Six Sigma makes the flowing process consistent.

In practice, many teams use DMAIC as the project frame and pick lean or statistical tools inside each phase. A value stream map often appears in Measure, and SMED or poka-yoke in Improve.

Training bodies certify Lean Six Sigma belts. The Lean Six Sigma certification page compares providers, and the Lean Six Sigma tools hub groups the software.

Which should you use first?

Start with lean when work waits, inventory piles up or lead times are long. These problems are usually visible without statistics.

Start with Six Sigma when output varies and nobody can explain why. Examples are scrap that comes and goes, or a test that sometimes fails.

Many teams do both in turn. They stabilize and simplify the process with lean, then use Six Sigma on the defects that remain.

For a wider comparison that includes kaizen, theory of constraints and other methods, see process improvement methodologies.

Software for each

Lean work is often paper-first, then moves to kaizen software, kanban software or value stream mapping software.

Six Sigma work needs statistics. Six Sigma software covers analysis tools, and SPC software monitors processes on the line.

Frequently asked questions