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Which RCA method should you use?
Four questions. You get the right method for your situation, what to pair it with, and whether it needs an 8D or A3 wrapper — with links to the free tool and template.
Four questionsMethod and its pairing8D / A3 wrapper adviceNo data collected
Answer all four questions to see your recommendation
How this selector decides
There is no single best root cause analysis method — only a best fit for four things: when you are looking (before or after the failure), what evidence you already hold, how the failure behaves, and who has to read the result. The logic below is the same one an experienced facilitator applies in the first five minutes of a session.
- Timing splits the field. Something failed → reactive methods (5 Whys, Fishbone, Pareto). Nothing has failed yet → proactive methods (FMEA, Fault Tree).
- Evidence picks the entry point. Counted data → rank first with Pareto. No data → widen first with a Fishbone. A firm hypothesis → drill straight down with 5 Whys.
- Stakes can override everything. If one specific outcome is catastrophic, or failures only occur in combination, you need the AND/OR logic of a Fault Tree regardless of the rest.
- Audience decides the wrapper, not the method. A customer or auditor means an 8D; internal alignment is often better served by a one-page A3.
The seven methods at a glance
| Method | Use when | Output | Typical time |
|---|---|---|---|
| 5 Whys | One suspected causal chain, need depth fast | Why-chain to one verified root cause | 30–60 min |
| Fishbone (Ishikawa) | Cause unknown, need breadth across categories | 6M / 6P cause map (hypotheses) | 1–2 h |
| Pareto | Many categories, counted data, must prioritise | Ranked chart, the vital few | 15–45 min |
| FMEA | Proactive risk screening before launch | RPN-scored worksheet + actions | 2–20 h |
| Fault Tree (FTA) | One catastrophic event, interacting failures | Logic tree, cut sets, probability | 4–40 h |
| 8D (wrapper) | Customer or auditor needs formal corrective action | D0–D8 report, contained and validated | 30–60 days |
| A3 (wrapper) | Internal alignment on one page | Single-sheet story, plan to follow-up | days–weeks |
Want the long-form trade-offs instead? See the full RCA tools comparison, or the head-to-head pieces: 5 Whys vs Fishbone, Pareto vs Fishbone, FMEA vs Fault Tree, 8D vs 5 Whys.
After you pick a method
Whatever the selector suggests, two things decide whether the analysis holds: a problem statement written as a measurable symptom rather than a theory, and a cause you verified with data before acting. Both failure modes show up in the five worked examples. When you are ready to document it, the free RCA report template covers the whole sequence in Excel, Word or PDF.
Frequently asked questions
Which root cause analysis method should I use?
It depends on timing, evidence, stakes and audience — the four questions above. Reactive plus a firm hypothesis points to 5 Whys; reactive with no idea points to a Fishbone; reactive with counted data points to Pareto first; proactive points to FMEA, or Fault Tree if a single outcome is catastrophic. A customer or auditor turns any of these into an 8D.
Can I use more than one method?
Yes, and most serious investigations should. Standard pairings: Pareto → 5 Whys (rank, then drill), Fishbone → 5 Whys (widen, then drill), FMEA → Fault Tree (screen, then dissect).
What is the difference between a method and a wrapper?
A method finds the cause (5 Whys, Fishbone, Pareto, FMEA, Fault Tree). A wrapper documents and governs the response: 8D adds containment, validation and customer reporting; A3 condenses it to one page for internal alignment. You always need a method; a wrapper only when someone else must read or audit the result.
When is 5 Whys not enough?
When the cause field is wide open (Fishbone first), when many categories compete and you have data (Pareto first), when failures only occur in combination (Fault Tree), or when it is chronic process variation rather than one chain — that is a DMAIC project, not a why-chain.
Is this selector free?
Yes — free, no signup, no email. It runs entirely in your browser and nothing is sent anywhere.