Root Cause Analysis with the 5 Whys: A Practical Guide
The 5 Whys is a simple technique for finding the underlying cause of a problem by asking "why?" repeatedly. Here is how to use it well, a worked example from a production line, and where it falls short.
The 5 Whys is a problem-solving technique associated with the Toyota Production System. When something goes wrong, you ask "why?" repeatedly, each answer forming the basis of the next question, until you reach a cause that, if fixed, prevents the problem from recurring. Five is a guideline, not a rule.
A worked example
Problem: a packaging line stopped for 45 minutes.
- Why did the line stop? The conveyor motor overloaded and tripped.
- Why did the motor overload? The conveyor bearing had seized.
- Why did the bearing seize? It was not lubricated.
- Why was it not lubricated? The bearing is not on the lubrication schedule.
- Why is it not on the schedule? The conveyor was extended last year and the new section was never added to the maintenance plan.
Replacing the bearing fixes today's stoppage. Updating the maintenance plan, and the process for adding new equipment to it, prevents the next one. See preventive vs predictive maintenance.
How to run a good session
- Go and see. Investigate at the place where the problem happened, with the people involved.
- Base each answer on facts, not assumptions. Check data, logs and the equipment.
- Focus on processes, not people. "Operator error" is rarely a root cause; ask why the process allowed the error.
- Follow branches. A problem may have more than one cause; follow each.
- Stop at a cause you can act on and that would prevent recurrence.
Limitations
The 5 Whys works well for relatively simple problems. Complex or high-risk issues with multiple interacting causes often need more structured tools, such as fishbone (Ishikawa) diagrams, fault tree analysis or formal incident investigation methods. Organisations such as the Lean Enterprise Institute publish further guidance on lean problem solving.
Close the loop
Record the problem, the causes found, the actions taken and who owns them, then check later that the problem has not returned. Tracking losses with OEE helps you choose which problems to investigate first.
