An Ishikawa diagram — also called a fishbone or cause-and-effect diagram — lays out every plausible cause of a problem, grouped into categories, so the search covers the whole field before anything is chosen.
The categories are conventional prompts rather than rules. Manufacturing typically uses the six Ms: method, machine, material, manpower, measurement, environment. Service work often substitutes people, process, policy, place. What matters is that the set forces attention somewhere it would not naturally go — the value is in the categories nobody wanted to fill in.
It is a structured brainstorm and produces hypotheses, not findings. The output is a list of candidates to test with data; treating the fullest branch as the answer is the most common misuse, because branch length measures how much the group knew about a category, not how much it contributes.
Built alone it inherits one person’s assumptions, which defeats the purpose. It wants the people who do the work, and it wants disagreement — a diagram everyone agreed on quickly has probably only recorded what was already believed.
