Key facts
- What lean adds: a focus on waiting, rework, excess stock, movement and other waste, seen through the customer's eyes.
- What Six Sigma adds: measurement and statistics to find and cut variation, aiming for about 3.4 defects per million opportunities.
- Where they meet: DMAIC projects that use tools from both.
- Origins: lean comes from the Toyota Production System; Six Sigma was developed at Motorola in the 1980s.
What does lean do inside a Six Sigma project?
Lean makes sure the project works on the right thing. A team can spend weeks analysing variation in a step that should not exist. Lean questions the step first: does the customer value it, and does work wait before or after it? Only then does the team measure and tighten what remains.
Where does lean fit in each DMAIC phase?
| Phase | What lean contributes |
|---|---|
| Define | Define value from the customer's point of view; map the flow with a value stream map |
| Measure | Measure lead time, cycle time and queues as well as defects |
| Analyse | Find the wastes of lean that sit behind the delay |
| Improve | Apply 5S, SMED, pull and flow, and poka-yoke |
| Control | Lock the new method in as standard work and visual controls |
What is the difference between lean and Six Sigma?
Lean asks "where is time and effort wasted?" and Six Sigma asks "why does the output vary?" A slow process that produces good parts needs lean first. A fast process that makes scrap needs Six Sigma first. Lean leans on maps, observation and quick trials; Six Sigma on measured data and control charts. Most problems need both.
Which comes first, lean or Six Sigma?
Usually lean. Removing obvious waste and stabilising the flow makes the process steady enough to measure, and statistics on an unstable process mislead. Variation work then goes after what remains. For the full method, belts and when to use it, see Lean Six Sigma; for the statistical side see Six Sigma tools.
Example
A distribution team found orders waiting a day for a packing check. A value stream map showed the delay was queueing between steps, which lean fixes by levelling work and removing hand-offs. Order errors then still averaged one in fifty, so a Pareto chart traced them to two product families and a check was added at the pick step.