Five Whys

Category: Risk identification & assessment · Reviewed by Matt Bartlett, Director · Founder · Last reviewed

Five Whys

The Five Whys is a simple iterative questioning technique used in root cause analysis. It was developed by Sakichi Toyoda and adopted as a core problem-solving tool within the Toyota Production System.

Method

Starting with a problem statement, the analyst asks “why?” and uses the answer to ask the next “why?”. After approximately five iterations the question typically reaches a systemic, actionable root cause. The number five is a heuristic — sometimes three is enough, sometimes seven.

Worked example — fleet motor claim

1. Why was the lorry damaged? Because the driver reversed into a bollard.
2. Why? Because they did not see it in the mirror blind spot.
3. Why? Because the reverse camera was broken.
4. Why? Because pre-shift checks were not completed.
5. Why? Because there is no enforced check-list and supervisors are not auditing compliance.

The action plan targets the systemic gap (5), not the driver (1).

Limitations

• Linear “why?” questioning can miss parallel contributing causes — pair with a fishbone diagram for complex incidents.
• Outcome depends heavily on the analyst’s framing of each “why”.
• Best used alongside, not instead of, fact-based investigation.

References

• Ohno, T. (1988). Toyota Production System: Beyond Large-Scale Production. Productivity Press.
• IEC 31010:2019, Annex B.16.

Cross-references

• Root cause analysis
• Fishbone diagram

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