What a fishbone is for in an 8D
An Ishikawa or fishbone diagram is a structured brainstorm. The effect from D2 sits at the head, and the team lists possible causes along six bones so nothing obvious is skipped. It is useful early in D4, when the team still disagrees about where to look.
The fishbone lists candidates; D4 still needs verification. A filled diagram is not a root cause. Reviewers often send back an 8D whose D4 is a fishbone picture with one bone circled and no data behind it. This worksheet adds a verification table so each candidate ends up confirmed, ruled out or still open, with the evidence written next to it.
The six categories (6M)
| Category | Questions to ask |
|---|---|
| Man (people) | Training, instruction clarity, shift handover, new or temporary staff. Ask what in the system let a person make the mistake. |
| Machine | Settings and drift, wear, maintenance status, tool or fixture condition, recent repairs. |
| Method | Work instruction, sequence, parameters, recent process or engineering changes. |
| Material | Supplier lot, material certificate, storage, mix-ups, a new sub-supplier. |
| Measurement | Gauge capability, calibration, sampling plan, how the defect is detected and why it wasn’t. |
| Mother Nature (environment) | Temperature, humidity, contamination, lighting, season or time of day. |
How to fill it in
- Head first: copy the measurable effect from D2, such as “short shots on cavity 4, 2–5 June, 38 parts”. A vague head gives a vague fishbone.
- Specific candidates: write something you could test, not a label. “Machine” is a category; “zone 3 above setpoint after the heater change” is a candidate.
- Use D2 to prune: if the defect is on cavity 4 only, candidates that would hit every cavity are less likely. Note which D2 fact supports or excludes each one.
- Verify: plan a test for each remaining candidate, record the result and mark it confirmed, ruled out or open.
- Escape too: also ask why the defect wasn’t detected. That needs its own cause, usually on the Measurement bone.
- Then drill down: take each confirmed cause into a 5 Whys chain to reach the system cause.
Writing up D4 from the fishbone? Get a free Accept / Conditional / Reject advisory on the 8D text, with missing evidence named, before it goes to the customer.
8D Gate is a free pre-send readiness advisory. It doesn't replace the customer's decision. We don't store your 8D body in our database; you can delete anytime. Your text is sent to Anthropic (Claude API) only to run the review; Anthropic doesn't use API data to train its models and by default deletes it within 30 days. Content flagged under its Usage Policy may be kept up to 2 years.
Weak vs strong: a filled example
Illustrative, fictional example from an injection-moulded part with short shots:
| Bone | Weak entry | Strong candidate |
|---|---|---|
| Man | Operator error | New operator on shift 3 skipped the purge step; the instruction has no purge check |
| Machine | Machine problem | Barrel zone 3 runs above setpoint after the heater change on 2 June |
| Material | Bad material | Resin lot R-418 moisture not checked before use; dryer log missing for 3 June |
| Measurement | Not detected | Visual check only; short shots under 1 mm not visible at the inspection lighting |
In the strong version, the verification table then shows the test: zone 3 set back and parts from both settings measured, with short shots only at the high setting. That result, not the diagram, is what goes into D4. The same structure works elsewhere: a food plant might list oven profile, ingredient lot and metal-detector sensitivity; an electronics line might list stencil wear, paste lot and AOI program. Every candidate still needs a test.
Common mistakes
- Circling one bone by vote and calling it the root cause.
- Category labels instead of testable candidates.
- “Operator error” on the Man bone with nothing behind it. See how to write a D4 root cause.
- No escape cause: the diagram explains occurrence only.
- Ruled-out candidates deleted, so the reviewer can’t see what was checked.
FAQ
- Is a fishbone diagram enough for D4?
- No. The fishbone lists candidates; D4 still needs verification. A cause goes into D4 when tests or data show it explains the defect, including where the defect doesn’t occur. Many teams use the fishbone to collect candidates and 5 Whys to drill into the confirmed ones.
- 6M or 4M or 8M?
- Use the categories your customer or procedure expects. 6M is common in manufacturing; service and office processes often use other headings. The categories are prompts, not a rule. Leave a category empty only after you have actually considered it.
- What does 8D Gate check?
- It gives a pre-send readiness check on the 8D text: an Accept / Conditional / Reject advisory with missing evidence and questions. It isn’t a customer decision. The review tool is v0.1: it reviews English 8D reports, and the interface and report are in English. Templates are also available in German, Czech and Polish.
Free pre-send readiness check
Before the 8D goes to the customer, run the text through the free supplier 8D readiness check.
Upload a PDF, Word (DOCX) or Excel 8D. You get an Accept / Conditional / Reject advisory, a score, section ratings, missing evidence, and questions to close gaps, with a PDF download. It is a pre-send readiness check for your team, not a customer decision.
8D Gate is a free pre-send readiness advisory. It doesn't replace the customer's decision. We don't store your 8D body in our database; you can delete anytime. Your text is sent to Anthropic (Claude API) only to run the review; Anthropic doesn't use API data to train its models and by default deletes it within 30 days. Content flagged under its Usage Policy may be kept up to 2 years.
Get the free 8D / D4 checklist
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