A fishbone diagram (also called an Ishikawa or cause and effect diagram) lists every possible cause of one problem, grouped into categories drawn as bones off a spine, with the problem written at the head. In manufacturing the categories are usually the 6 Ms: Man, Machine, Method, Material, Measurement and Mother Nature. The diagram gives you a complete list of suspects; data then tells you which one did it.
Free fishbone diagram templateHow to make oneCompleted exampleFishbone vs 5 Whys
What Is a Fishbone Diagram?
The fishbone diagram is a structured brainstorm for one problem. The effect, a measured problem, goes in the head on the right. A horizontal spine runs back from it, and each major bone is a category of cause. Possible causes hang off each bone, and the reasons behind them hang off those as smaller bones. Kaoru Ishikawa made it popular in Japanese quality circles in the 1960s, and it is one of the seven basic quality tools along with the check sheet, Pareto chart, histogram, scatter diagram, control chart and flowchart.
Its value is breadth. Left alone, a team fixes on the first plausible cause, often "operator error". The categories force the team to look at the machine, the method, the material, the gauge and the surroundings too. The glossary entry has the one-line definition.
Fishbone Categories: 6M, 8P and 4S
| Set | Categories | Use it for |
|---|---|---|
| 6M | Man (people), Machine, Method, Material, Measurement, Mother Nature (environment) | Production, quality and maintenance problems |
| 8M | The 6 Ms plus Management and Maintenance | When decisions or upkeep are likely suspects |
| 8P | Product, Price, Place, Promotion, People, Process, Physical evidence, Productivity | Service, sales and marketing problems |
| 4S | Surroundings, Suppliers, Systems, Skills | Service delivery and supplier problems |
The categories are prompts, not rules. If a problem clearly needs a "Software" or "Packaging" bone, add one. The free fishbone diagram template switches between the 6M, 8P and 4S sets.
How to Make a Fishbone Diagram: 7 Steps
A session takes 30 to 60 minutes with four to eight people who know the process, including at least one who works it every day. Hold it next to the process if you can.
Fishbone Diagram Example: Leaking Valve Assemblies
An illustrative example, not a client case. A line assembles pneumatic valves and leak-tests every one. Problem (the head): leak test failures on valve line 2 rose from 0.7% to 2.0% from week 20. The team ran a one-hour session and drew this:
Swipe the diagram sideways to see all of it.
Checking the causes against data
The team voted, and the top four suspects were the new night assemblers, the shorter leak test, the new O-ring lot and the master leak check. Each was checked by splitting the same records a different way:
| Suspect | How it was checked | Result |
|---|---|---|
| New night assemblers | Failure rate by shift since week 20 | Nights 2.0%, days 1.9%: no real difference. Not the cause. |
| Leak test cut from 30 s to 20 s | Retested 50 passed valves at 30 s | All 50 passed again. Not the cause. |
| Master leak not run daily | Ran the master leak on the tester | Tester flagged it correctly. Not the cause. |
| New O-ring lot | Failure rate by O-ring lot, same line, same weeks | New lot: 99 fails in 3,400 = 2.9%. Old lot: 18 in 2,600 = 0.7%. Confirmed. |
Check of the totals: (99 + 18) ÷ (3,400 + 2,600) = 117 ÷ 6,000 = 1.95%, the 2.0% in the problem statement. The old lot runs at the old rate, so the whole rise comes from the new lot. A Pareto chart of failures by lot would show the same thing in one picture.
From the verified cause to a root cause
The fishbone stops at "new O-ring lot". That is a cause, not yet something you can fix for good, so the team ran a 5 Whys on it:
- Why do new-lot O-rings leak? Their cross-section is at the low end of tolerance, so the squeeze in the groove is below the design minimum.
- Why was low-end material accepted? Incoming inspection checks hardness only, not cross-section.
- Why only hardness? The inspection plan was written when there was one supplier with a very stable process.
- Why was it not updated for the new supplier? The supplier approval process does not trigger a review of the incoming inspection plan.
Actions: contain (quarantine the new lot, sort by cross-section), correct (add cross-section to the incoming plan), prevent (add "review incoming inspection plan" to the supplier change checklist). Then watch the leak rate for 30 days.
Fishbone vs 5 Whys
| Fishbone diagram | 5 Whys | |
|---|---|---|
| Gives you | Breadth: all plausible causes in categories | Depth: one chain from symptom to root cause |
| Best when | Many possible causes, nobody sure where to look | The likely cause is known or verified |
| Output | A list of suspects to check with data | A root cause you can act on |
| Time | 30 to 60 minutes with a team | 15 to 30 minutes, often at the process |
| Main risk | Treating the brainstorm as proof | Following the wrong branch with confidence |
They work best together, as in the example: fishbone for the suspects, data to pick the culprit, 5 Whys to get underneath it. 5 Why Pro keeps both in one offline file: brainstorm on a fishbone, then drill each cause into a branching why tree with evidence on every answer. For how the fishbone fits with 8D, A3 and fault tree analysis, see root cause analysis methods compared.
Common Fishbone Mistakes
Do
- Write a measured problem: what, where, how much, since when
- Include people who run the process every day
- Write silently first, then discuss
- Check the top causes against data before acting
- Take verified causes into 5 Whys and actions
Avoid
- A vague head such as "quality issues"
- Filling bones with solutions ("need new fixture") instead of causes
- Stopping at "operator error" or "training"
- Fixing the cause with the most votes without checking it
- Leaving a bone empty without looking there
Fishbone Diagram FAQ
What is a fishbone diagram?
A fishbone diagram is a cause-and-effect chart that lists the possible causes of one problem, grouped into categories drawn as bones off a central spine, with the problem written at the head. Kaoru Ishikawa popularized it in Japanese quality circles, so it is also called an Ishikawa diagram, a cause and effect diagram or a herringbone diagram.
Is a fishbone diagram the same as an Ishikawa diagram?
Yes. Fishbone diagram, Ishikawa diagram, cause and effect diagram and herringbone diagram are four names for the same tool. Ishikawa is the name of the quality engineer who made it popular; fishbone describes its shape.
What are the 6 Ms of a fishbone diagram?
Man (people), Machine, Method, Material, Measurement and Mother Nature (environment). They are the usual categories for a manufacturing problem. Some teams add Management and Maintenance to make 8 Ms.
What are the 8 Ps and the 4 Ss?
The 8 Ps suit service and sales problems: Product, Price, Place, Promotion, People, Process, Physical evidence and Productivity. The 4 Ss suit service and supplier problems: Surroundings, Suppliers, Systems and Skills. Use whichever set makes the team look in places it would otherwise skip.
How many causes should each bone have?
There is no fixed number; three to six per category is typical for a one-hour session. An empty bone is a prompt to look harder there, and a bone with fifteen causes usually means the problem statement is too broad.
What comes after a fishbone diagram?
Verification. A fishbone lists possible causes; it proves nothing. Pick the two or three most likely causes, check them against data (by shift, by lot, by machine, by time since a change), then run a 5 Whys on each verified cause to reach something you can fix, and write actions with owners and dates.
What is the difference between a fishbone diagram and 5 Whys?
A fishbone gives breadth: every plausible cause across all categories. 5 Whys gives depth: it follows one cause down to a root you can act on. Use the fishbone first when a problem could have many causes, then 5 Whys on the causes the data confirms.