Management & Thinking
Pro5 Whys
5 Whys 五問追因 · Source: Sakichi Toyoda · Toyota Production System
Incidents, bugs, customer complaints, team friction — any problem you want to trace to root cause, not stop at surface symptom
Core Concept
Invented by Sakichi Toyoda, central to the Toyota Production System: ask "why" five times about a problem, each answer becomes the target of the next "why." Designed to cut through symptom-layer, behavior-layer, and process-layer to reach where you can actually fix things — usually a system design flaw, not "someone wasn't paying attention." Common failures: asking five "whos" (witch-hunt), stopping at layer 2–3 (still symptom), answering with opinion rather than fact. Five is a convention; the real target is reaching a layer you can change.
Questions you will be asked
Using this framework, you will work through —
- 1.Describe what happened. Facts, not evaluations.
- 2.Why 1: Why did this happen?
- 3.Why 2: Why did the previous cause occur?
- …and 3 more
FAQ
Why "5" whys, not 3 or 7?
"5" is a median Toyota observed in 1950s factory work — most problems reach root cause around the fifth iteration. But it's an empirical observation, not a rule: simple problems bottom out at 3; deep systemic ones take 7–8. The real stop criterion is whether the next answer is at a level the organization can actually control (design a process, change structure, make a decision) — not a fixed count.
How is 5 Whys different from a fishbone diagram?
5 Whys digs vertically — one line down to the root. Fishbone (Ishikawa) fans horizontally — listing causes across categories (people, machine, material, method, environment) at once. 5 Whys fits situations with a clear primary cause (production defect, customer complaint); fishbone fits situations where the direction itself is unclear (sales decline, churn spike). Typical sequence: fishbone first to find the line worth chasing, then 5 Whys down that line.
How do I avoid stopping at "human error" as the root cause?
Stopping at "person X wasn't careful" is the most common way 5 Whys fails — it feels like an answer, but it's really just the doorway to the next Why. Technique: when you reach "the person," ask again — why would this person make this error? Inadequate training? Process designed to invite errors? Wrong incentives? Bad tooling? Those are the levels organizations can change. Toyota's principle: "If the same person wouldn't make this error in a different system, the root isn't in the person."
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