Effective corrective action depends on finding the true root cause of a problem. Two tools are used more than any other: the 5 Whys and the fishbone (Ishikawa) diagram.
5 Whys
Ask “why?” repeatedly until you reach a cause that, if removed, would prevent recurrence. It is fast and works well for simple, linear problems.
- Strengths: quick, needs no special training
- Watch out for: following a single path and missing contributing causes
Fishbone diagram
The fishbone diagram organizes possible causes into categories — commonly people, methods, machines, materials, measurement and environment.
- Strengths: encourages a team to consider many causes
- Watch out for: long lists of opinions without data
Combine them
Use the fishbone to explore possible causes, verify the most likely ones with data, then use 5 Whys to drill down to the root cause. Always check that the corrective action addresses the root cause — not just the symptom.
Root cause analysis is covered in the CQE and Lean Six Sigma Green Belt certifications.