The method · 10 December 2022
What is Lean Six Sigma — and when is it the wrong tool?
The method works. The hype around it often does not. What Lean Six Sigma actually promises, in plain English — and where it earns or loses its keep.
Lean Six Sigma is two ideas joined together. Lean asks: where does work sit around waiting, and which steps add nothing? Six Sigma asks: why does the same process give different results on different days, and how do we make it dependable? Both are useful. Together they're better. But let's look at them honestly instead of just repeating the brochure.
Chapter 1 · The method
The famous number, explained
Six Sigma's name comes from a target: a process so consistent that it produces only about 3.4 mistakes in every million chances to make one. Written as a formula:
In plain words: count your mistakes, divide by how many chances you had to make one, and scale it up to "per million" so different processes can be compared fairly. That's all DPMO is — a mistake rate with the units tidied up.
The 3.4 target assumes a lot, though: that your process is stable, that you measure it honestly, and that you count "chances for a mistake" fairly. None of that comes free.
Chapter 2 · The method
Where it genuinely helps
- Work that repeats — claims, orders, lab tests, deliveries — where you can actually count mistakes and delays.
- Problems where everyone has a different opinion about the cause. The method's rule is simple: measure first, argue after.
- Teams willing to hear that their favourite theory is wrong.
Chapter 3 · The method
Where it goes wrong
The failures are rarely about the maths. Training courses become certificate factories — people learn the vocabulary but never fix a real process. The mistake-counting gets gamed: if you double the number of "chances" on paper, your mistake rate halves without anything improving. And applying Six Sigma to one-off creative work misses the point entirely — a process has to repeat before you can measure how consistent it is.
Chapter 4 · The method
The honest test
Before starting, ask three questions. Does the work repeat? Can we count the mistakes? Will we accept what the numbers say, even if we don't like it? Three yeses and the method will pay for itself. Any no — fix that first.
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