Root cause analysis: 5 Why and Ishikawa

01Ishikawa: map the possible causes

An Ishikawa or fishbone diagram organises possible causes into categories, commonly the 6M: Man (people), Machine, Method, Material, Measurement and Milieu (environment). Its purpose is breadth: making sure the team considers all plausible causes before narrowing down.

Build it with people who know the process, ideally at the workstation. Then mark which causes are supported by evidence.

025 Why: follow one cause chain to the end

The 5 Why technique asks 'why' repeatedly until a cause is reached that, if removed, prevents recurrence. Five is a guideline, not a rule.

Example, machined bore oversize: Why oversize? Boring bar deflected. Why? Tool overhang longer than standard. Why? A longer bar was used because the standard bar was broken. Why was a non standard bar allowed? No tool list in the set-up sheet. Why? Set-up sheets do not specify tool assemblies for this machine family. Action: tool assemblies specified and checked at set-up approval.

03Verify before acting

A cause is a hypothesis until verified. Verification methods include reproducing the defect by switching the cause on and off, comparing good and bad parts or periods (is / is not), checking data such as SPC charts or parameter logs, and physical analysis such as metallography for material problems.

04Common mistakes

• Stopping at a symptom ('dimension out of tolerance').
• Jumping to blame people instead of asking why the system allowed the error.
• Branching 5 Why chains that mix several problems.
• Ignoring the detection side: why did inspection not catch it?
• No verification of the proposed cause.

05Prioritise with data

When many causes or defect types exist, use a Pareto analysis to focus on the vital few.