5 Whys
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5 Whys is a root cause analysis technique that asks 'why' repeatedly to trace symptoms back to fundamental causes.
Definition
5 Whys is a root cause analysis technique that iteratively asks "Why?" to peel back layers of symptoms and reach fundamental causes. Starting with a problem statement, each answer becomes the subject of the next "Why?" question. The number five is a rule of thumb—sometimes fewer suffice, sometimes more are needed. The goal is to move from symptoms (which recur if only treated) to root causes (which, when addressed, prevent recurrence). 5 Whys works best for simple cause-and-effect chains; complex problems may require additional tools.
Examples
Problem: Parts are out of specification. Why? The machine produced oversized parts. Why? The cutting tool was worn. Why? Tool changes weren't made on schedule. Why? There's no tool life tracking system. Why? We assumed operators would notice wear. Root cause: lack of systematic tool management.
Key Points
- Five is a guideline, not a rule—go until you reach actionable root causes
- Each "why" should have factual, verified answers
- If answers are assumptions, verify before proceeding
- Multiple branches may exist; explore significant ones
Common Misconceptions
5 Whys always finds one root cause. Many problems have multiple contributing causes. 5 Whys may branch at various levels; significant branches should all be explored.
5 Whys works for all problems. 5 Whys works best for simple cause-effect chains. Complex systems with multiple interacting factors may require more sophisticated analysis tools like fishbone diagrams or fault trees.