Fishbone diagram
The fishbone (Ishikawa) diagram sorts every possible cause of a problem into a small set of categories branching off a spine, so a team searches for the cause systematically instead of guessing.
Six category branches fan out toward a single problem, the shape of the fish itself.
Reach for this when…
- A defect keeps recurring and everyone has a different theory why.
- You are about to fix the obvious cause without checking it is the real one.
- A cross-functional team needs one shared picture of what could be going wrong.
How to run it
- Write the problem clearly at the head of the fish.
- Pick your cause categories, commonly Machine, Method, Material, Man, Measurement, Environment.
- Brainstorm causes under each category without debating them yet.
- Ask 'why' again on the causes that keep coming up, to go one layer deeper.
- Circle the two or three most likely root causes, and test them.
A worked example
Situation. Siobhan Byrne ran quality at Lee Valley AutoParts, a components manufacturer in Cork, Ireland, where a batch of brake housings kept failing tolerance checks and three departments blamed each other.
Applied. Her team ran a fishbone session across the six standard categories, and the Machine branch surfaced an overlooked cause: calibration drift on one specific press, showing up only on the night shift.
Result. Recalibrating that press weekly instead of monthly cut the failure rate on that line by more than half.
The catch
The diagram generates causes well but does not rank them; teams often walk away with twenty plausible causes and no way to prioritise without a follow-up tool such as Pareto analysis. It is also only as good as who is in the room, since a fishbone run by one department misses causes that live in another.
A full fishbone with nothing circled at the end is a brainstorm, not an analysis. It has to end in a shortlist you actually test.
Origin: Kaoru Ishikawa