Abstract
The first explanation for a problem is usually a symptom. If you ask why it happened, then ask why about that answer, you work back along the chain toward something you can change. Five is a rule of thumb. You stop when the next answer would be out of your hands or wouldn't help.
The method comes from the Toyota Production System, where it was used on the shop floor right after a defect turned up. It works best when the facts are fresh and the questions are about how the process failed.
Method
- 01
Describe the problem precisely
Say what happened, where and when, in one sentence. A specific problem leads to specific causes.
- 02
Ask why it happened
Write the most direct cause you can back up with evidence. Guesses don't count.
- 03
Ask why again
Treat that answer as the new problem and ask why. Keep going as long as each answer rests on facts.
- 04
Stop at something you can fix
When you reach a cause in a process or system you can change, that's your root cause for now.
- 05
Act and check
Pick a countermeasure, give it an owner, and check later whether the problem has come back.
When to use it
- A problem keeps coming back and you're tired of patching the symptom.
- Right after an incident, while people still remember the details.
- As a quick first pass before a fuller investigation.
Common pitfalls
- Stopping at a person. "Someone forgot" isn't a root cause. Ask why the process let it happen.
- Following one chain when there are several causes. Complicated problems often need a fishbone diagram.
- Answering with assumptions. Check each answer against data before asking the next why.
- Going so deep you end up at human nature or the economy, which nobody can fix.
Origin and references
Attributed to Sakichi Toyoda, developed by Taiichi Ohno at Toyota (1930s).
- Ohno, T. (1988). Toyota Production System: Beyond Large-Scale Production. Productivity Press.
- Liker, J. K. (2004). The Toyota Way. McGraw-Hill.