Module 4 · Lesson 4 of 22
Five Whys and cause validation
This lesson and the entire Lean Six Sigma Black Belt academy are free of charge — no fees, ever.
Learning objectives
- Trace a causal chain from event to controllable system cause.
- Ask mechanism-focused ‘why’ questions.
- Validate each link with evidence.
Five Whys done well
The number five is flexible. Each ‘why’ answer must be specific, observable and supported by records, experiment or direct observation.
Branch when the evidence supports multiple contributing causes; a single chain rarely explains a real defect.
Garment-factory example
Broken stitches → needle damage → missed replacement interval → no cycle-based standard → no ownership or visual trigger. Each link is confirmed by needle-change logs and gemba observation.
Method
- Start from the specific event.
- Ask why it occurred and record evidence.
- Branch when multiple causes appear.
- Stop at a controllable system cause, not blame.
Common mistakes
- Ending with ‘carelessness’.
- Forcing one linear chain when several exist.
Knowledge check
Pick one answer per question. Explanations appear after you submit.
1. A Five Whys link is credible only when:
2. Ending with ‘operator carelessness’ is:
Author: NOSK.life · Last reviewed: 2026-07-21