D4 — Identify and verify root cause
D4 is where the investigation earns its keep. The goal is not to find a plausible cause — it is to prove the root cause and the escape cause with evidence a peer can reproduce.
Key takeaways
- Start from D3 — never brainstorm causes from scratch.
- Generate candidate causes; do not stop at the first plausible one.
- Reject any cause that does not explain the Is-Not.
- Verify by reproduction — cause it, then eliminate it.
Generating candidate causes
Use Ishikawa (man, machine, method, material, measurement, environment) or a fault tree to fan out candidates. Score each candidate against D3: does it explain both the Is and the Is-Not?
Proving the root cause
The strongest proof is reproduction: reintroduce the suspected cause under controlled conditions and observe the defect return; then eliminate it and observe the defect go away. If reproduction is not possible, use the strongest available evidence — designed experiments, historical data correlation, part disassembly.
The escape cause
Even a proven root cause is not enough. Ask: why did our controls not catch it? The escape cause usually points at PFMEA gaps, control plan weaknesses or ineffective detection.
Common mistakes
- Accepting the first plausible cause without disproving alternatives.
- Treating a 5 Why chain as proof.
- Forgetting the escape cause — fixing production without fixing detection.
How Qhubio supports D4
Qhubio's D4 phase retrieves relevant chunks from your evidence documents and flags conflicting or unsupported facts — so root cause candidates stay grounded in real evidence.
- Evidence-grounded retrieval across engineering documents.
- Conflict and unknown-fact flags in the reasoning panel.
- Root cause candidates linked to the source evidence.
Frequently asked questions
- How many candidate root causes should we generate?
- Enough to feel uncomfortable with your first guess. Teams that stop at 2–3 typically miss the real cause.
- What if we cannot reproduce the defect?
- Use the strongest available alternative evidence and document the confidence level. Do not claim a proof you do not have.
- Do we need to fix the escape cause separately?
- Yes. Root cause and escape cause typically lead to different corrective actions — one in the process, one in the control system.
Related guides
Run this D-step inside a structured workspace
Qhubio is a structured 8D workspace: evidence-grounded phases, phase-gated approvals and audit-ready exports.