What is a Fishbone Diagram (Ishikawa)?
The fishbone (Ishikawa, cause-and-effect) diagram puts one problem at the fish's head and organizes potential causes along the bones — classically people, process, equipment, materials, environment, and measurement. The team brainstorms down each bone, then digs "why?" until root causes surface.
Its power is structure: unstructured blame sessions produce the loudest theory; the fishbone forces the quiet categories (measurement, environment) to get examined too. Pair it with the five whys and you have the core root-cause toolkit.
Worked example
Concrete cylinders keep failing strength tests. The fishbone session works each bone: people (new batching operator), materials (aggregate supplier changed), equipment (mixer calibration overdue), environment (poured during a heat wave). Testing reveals it's the aggregate — the argument everyone hadn't been having.