Lean and quality teams use RCA after defects, breakdowns, safety incidents and customer complaints. It sits between containment, which protects the customer today, and the corrective action that protects them for good.
Where it comes from
RCA draws on several quality traditions: the five whys from Sakichi Toyoda and Toyota, the cause-and-effect diagram from Kaoru Ishikawa, and the problem-solving formats used in Six Sigma and the automotive 8D method.
At a glance
Below the waterline
Swipe sideways to see the whole diagram.
How to apply Root cause analysis
- 1
Define the problem
Write one clear problem statement: what happened, where, when and how big, compared with the standard. Avoid naming a cause in the statement.
- 2
Contain it
Protect the customer while you investigate. Quarantine suspect parts, add a temporary check or switch to a backup process.
- 3
Gather facts at the gemba
Go where it happened, look at the parts, the equipment and the records, and talk with the people involved. Collect data before forming theories.
- 4
List possible causes
Use a fishbone diagram to list causes across method, machine, material, measurement, environment and people. Keep the list broad at this stage.
- 5
Drill down and verify
Ask why for the most likely causes until you reach one you can act on. Then prove it with data or by turning the problem on and off.
- 6
Fix and prevent
Put in a corrective action that removes the root cause, ideally with mistake-proofing. Update standards and check other lines with the same risk.
- 7
Confirm it worked
Watch the results over enough time to be sure the problem is gone. Close the RCA only when the data shows it.
Worked example
A bottling line rejects caps several times a week. The team contains the issue with a torque check, then finds through a fishbone and five whys that one capper head wears faster because it gets no lubrication on the PM route. Adding the head to the route and a wear gauge to the checklist stops the rejects, and the plant adds the same check to its other capper.
Common mistakes
- Stopping at human error as the root cause, instead of asking why the process allowed the error.
- Picking a cause in the meeting room and never verifying it with data or at the gemba.
- Closing the RCA when the action is assigned, not when results show the problem has stopped.
When Root cause analysis is not the right tool
A full RCA takes time. For a one-off, low-risk issue with an obvious cause, a quick fix and a note may be enough.
RCA also needs facts. If nobody can see the process or the parts, start by collecting data rather than running a meeting.
Root cause analysis methods compared
The five whys suits simple problems with one chain of cause. The fishbone diagram helps when many causes are possible. Pareto analysis shows which problem to tackle first.
For a full investigation, teams wrap these tools in a format: an A3 in lean, DMAIC in Six Sigma, or 8D in automotive supply chains. The root cause examples guide walks through common cases.
| Method | Best for | Output |
|---|---|---|
| Five whys | One clear chain of cause | A cause you can act on |
| Fishbone diagram | Many possible causes | A grouped list of causes to test |
| Pareto chart | Choosing which problem to tackle | The few causes behind most defects |
| Fault tree analysis | Complex failures with combined causes | A logic tree of failure paths |
| 8D | Customer complaints in supply chains | A documented eight-step report |