A fishbone (Ishikawa) diagram lists every possible cause of a problem under a handful of headings so your team looks widely before settling on one. The common headings in manufacturing are man, machine, method, material, measurement and environment. The diagram does not find the root cause. It gives you a list of suspects. You find the cause by checking each one against evidence, then taking the survivors into a 5 Why.
What it is for
The fastest answer in the room is usually wrong. A fishbone slows that down on purpose. It makes the team write down everything that could have caused the problem, in every area, before anyone picks a favorite. Kaoru Ishikawa popularized the diagram in Japan, and it is one of the oldest quality tools still in daily use.
The six headings
| Heading | What goes under it | Example |
|---|---|---|
| Man (people) | Training, skill, instructions followed, staffing | New operator on the machine, step skipped |
| Machine | Equipment, tooling, fixtures, maintenance | Worn drill, spindle runout |
| Method | The procedure, setup, sequence, work instruction | Feed rate changed, setup sheet out of date |
| Material | Raw material, components, packaging, supplies | New lot of bar stock, new tote design |
| Measurement | Gauges, inspection methods, sampling | Gauge worn, wrong gauge used |
| Environment | Temperature, humidity, cleanliness, layout | Cold shop in the morning, dust |
Some plants use four or eight headings, or their own. Use what fits your process. What matters is that the headings make people look in places they would not otherwise look.
How to build one
Write the problem at the head
Use your problem statement. A vague head gets a vague fishbone.
Draw the spine and the main bones
A long arrow pointing at the problem, with one bone for each heading.
Get the right people in the room
The operator, the setup person, maintenance, the inspector and quality. People who do the work know where it goes wrong. A fishbone drawn by one engineer at a desk is a guess.
Brainstorm causes for each bone
Everyone writes causes on sticky notes, one per note, and places them on the bone. No arguing, no ranking, no “that can’t be it” yet.
Ask why under each cause
For a cause such as “drill worn,” add the next level down: why was it worn, how is tool life tracked? These become the smaller bones.
Check every cause against evidence
This is the real work. Pull the records, look at the parts, test it. Cross out what the evidence rules out and write down the proof next to each one you cross out.
Take the survivors into a 5 Why
What is left are your likely causes. Dig into each with a three-legged 5 Why to reach something you can change.
A made-up example: burrs on drilled holes
In 8 of 120 parts from one lot, the cross hole has a burr over the limit. The team builds a fishbone and then checks each cause.
| Possible cause | What the team checked | Result |
|---|---|---|
| Drill worn (machine) | Tool life log | Drills changed at 40 percent of the planned life. Ruled out. |
| Coolant concentration (environment) | Daily coolant log | Within range every day. Ruled out. |
| Material hardness (material) | Cert and a hardness check | Same heat as good lots. Ruled out. |
| Operator changed feed rate (method) | Program history and setup sheet | Program unchanged. Ruled out. |
| Gauge reading wrong (measurement) | Checked burrs with a second gauge | Gauge agrees. Ruled out. |
| New drill regrind vendor (material) | Regrind records and a point angle check | Drills from the new vendor have a different point angle. Likely. |
One cause survived, and it came from a place nobody would have blamed first. A 5 Why from there leads to how the vendor was changed, and why nobody checked the regrind specification when it was.
Mistakes that waste the session
- Stopping at the brainstorm. A fishbone full of possible causes with no evidence is just a longer opinion.
- Voting on the answer. Causes are true or not true. Evidence settles it, not a show of hands.
- Too few people, or the wrong ones. If the person who runs the machine is not there, half the bones are missing.
- A vague head. “Poor quality” gives you nothing to hang causes on.
- Listing people as causes. Under man, write what in the process let a mistake happen, not who made it.
- Leaving it on the wall. File the diagram with the evidence checks. The next person on this problem needs them.
The Fishbone Diagram (QA-F-365) workbook sorts causes into the six headings and keeps the evidence next to each one.
Questions people ask
What is the difference between a fishbone and a 5 Why?
A fishbone lists possible causes across the whole process. A 5 Why follows one cause down to something you can change. Use the fishbone first, then the 5 Why.
Do I have to use six headings?
No. Six is common in manufacturing. Four, eight or your own headings work if they help the team look broadly.
How many people should be in the session?
Four to eight is a good range, always including someone who runs the process every day.
How long should it take?
The drawing takes under an hour. The evidence checks take as long as the problem needs. That second part is the work.
Is a fishbone enough for an 8D?
It helps, but customers want the evidence. Show which causes were checked and ruled out, and the proof for the one you kept.