5 Whys
Problem: The packaging machine suddenly stops working.
- 1
Why the packaging machine suddenly stops working?
The fuse has blown.
- 2
Why the fuse has blown?
The machine was overloaded.
- 3
Why the machine was overloaded?
There were too many products on the conveyor belt.
- 4
Why there were too many products on the conveyor belt?
The sensor that counts the products is faulty.
- 5
Why the sensor that counts the products is faulty?
The scheduled maintenance of the sensor was not carried out.
Root cause
Check: read the chain back
Because the scheduled maintenance of the sensor was not carried out, and so the sensor that counts the products is faulty, and so there were too many products on the conveyor belt, and so the machine was overloaded, and so the fuse has blown, and that is why: the packaging machine suddenly stops working.
If this sentence does not flow, there is a gap or a leap in the reasoning. Every step must follow logically from the previous one.
Root cause found, with a countermeasure.
Check in a few weeks whether the problem has stayed away. If it comes back, there was another why underneath. Does your fishbone diagram have more marked causes? Explore each of them here in its own line.
How 5 Whys works
- Start with the problem as you see it happening. A fact, as concrete as possible.
- Ask why and write the answer down as a cause: "because ...". Then ask why again about that answer. Five times is a guideline, not a rule: sometimes you are there after three, sometimes after seven.
- Read the chain back, from the bottom up: because A, and so B, and so C. If that sentence does not flow, you have skipped a step or made a leap.
- Stop at the cause you can act on and think of a countermeasure that removes the cause, not one that asks people to pay more attention.
When do you use it?
In the Analyse phase, as soon as you have a cause that needs to be explored further: a marked cause from your Ishikawa diagram, a Kaizen burst on your process map, or simply a failure that keeps coming back. Do it as a team: different people see different things, and together you get to the core faster. It also works as a way of probing while you map a process.
Common mistakes
- Looking for someone to blame. "Because Pete was not paying attention" is a judgement, not a cause. What made it possible for this to happen?
- Stopping too early. The first two answers are almost always still symptoms.
- Going too far. Whoever ends up at "the economy" or "the culture" is past the point where they can still change anything.
- Writing solutions down as causes. "Because there is no checklist" is the solution backwards. What happens now?