Lean teams use it because it is quick and needs little training or software. It pushes teams to fix the process instead of blaming people or patching symptoms.
Where it comes from
The technique is credited to Sakichi Toyoda and was promoted by Taiichi Ohno within the Toyota Production System.
At a glance
Asking why until the cause is fixable
Swipe sideways to see the whole diagram.
How to apply 5 Whys
- 1
Define the problem precisely
Write a specific, factual problem statement. Include what happened, where and when.
- 2
Gather the right people
Include people who do the work and saw the problem. Go to the gemba if you can.
- 3
Ask why and record the answer
Answer with facts you can verify. Each answer becomes the subject of the next why.
- 4
Keep going until it is actionable
Five is a guide, not a rule. Stop when you reach a process cause you can control and fix.
- 5
Verify the chain
Read the chain backward using "therefore" to check the logic. If a link fails, revisit it.
- 6
Act and follow up
Put a countermeasure on the root cause and confirm the problem does not return.
Worked example
A machine stopped because a fuse blew. The fuse blew because the motor overloaded, which happened because a bearing lacked lubrication. The lube pump intake had no strainer and clogged, so the fix is adding a strainer and a check to the PM schedule.
Common mistakes
- Stopping at human error, such as "the operator forgot", instead of asking why the process allowed it.
- Following a single chain when the problem has several contributing causes.
- Accepting guesses as answers without checking them against data or the gemba.
When 5 Whys is not the right tool
Five whys follows one chain of cause. Problems with several interacting causes need a fishbone diagram or a fuller investigation.
The answers also depend on who is in the room. Check each answer with data or at the gemba before you act on it.
Branching whys
When an answer has more than one cause, split the chain. Each branch gets its own series of whys, which turns the list into a small tree.
Stop when you reach a cause you can act on that would prevent the problem returning. The number five is a guide, not a rule.