Root Cause Analysis (5 Whys and Fishbone)

How to use the Root Cause Analysis

Root cause analysis looks past the symptom to the cause that, if removed, stops the problem coming back.

1. State the problem precisely: what, where, when and how much.
2. Use the fishbone to brainstorm possible causes under Man, Machine, Method, Material, Measurement and Environment.
3. Take the most likely causes and ask 'why?' repeatedly until you reach something you can act on.
4. Verify the cause with data before committing to a fix.

Worked example

Problem: 3% of brackets have oversize holes. Why? The drill is worn. Why? It is replaced only when it breaks. Why? There is no tool life limit. Root cause: no tool-change standard. Countermeasure: replace the drill every 2,000 holes and record it on the daily equipment check.

Common mistakes to avoid

• Stopping at 'operator error', which is rarely a root cause.
• Following one chain of whys when there are several contributing causes.
• Not testing the cause before spending money on a fix.

Frequently asked questions

Do you always ask 'why' exactly five times?

No. Five is a guide. Stop when you reach a cause that you can control and that explains the problem.

What is a fishbone diagram?

Also called an Ishikawa or cause-and-effect diagram, it groups possible causes into categories so nothing obvious is missed.

When should I use root cause analysis?

For any recurring problem, customer complaint, safety incident or quality defect where a quick fix has not stopped it.