Problem solving

Ishikawa / fishbone diagram

In French: Diagramme d’Ishikawa (5M)

The Ishikawa diagram lays out in a fishbone all the possible causes of a problem, grouped by family, so as not to stop at the first plausible hypothesis.

What it is

Attributed to Kaoru Ishikawa, the cause-and-effect — or fishbone — diagram places the effect on the right, at the head, and traces the causes back along bones grouped by family. Its function is to structure the search, not to name a culprit.

The families most used are the 5M: manpower, material, machine, method, mother nature (environment). Measurement is commonly added — the measurement system is a source of variation in its own right, and forgetting it means looking for a problem that may exist only in the instrument. Some add management or money; the list is in no way prescriptive, it serves to leave no blind spot.

The diagram is built as a group, with those who do the work. A family filled in by a single person reflects what that person already knows.

What it does not do

An Ishikawa produces hypotheses, never conclusions. A cause written on a bone is not demonstrated: it is a candidate. The passage from candidate to proof is made through data — counting, comparison, statistical test — and it is that passage that distinguishes a Six Sigma project from a well-run brainstorming meeting.

To dig into a branch, the complementary tool is the repeated question "why?". It goes down from a symptom to a mechanism, where the Ishikawa sweeps across the width. The two complement each other: width first, depth next.

The common mistake

Filling in the five families for form’s sake, then retaining the cause one had in mind before starting. It is the most widespread use of the tool, and the most useless: it gives an intuition the appearance of a method. The tell-tale sign — if no data went to decide between two branches, the analysis did not take place.

On the shop floor — Mechanical engineering

On a surface-finish defect, the team had converged in ten minutes on tool wear. The complete diagram brought out an "environment" branch nobody had considered: the workshop temperature, which varied by nine degrees between the start and the end of a night shift. The data decided in favour of that branch.

The other tools illustrated by this trade: machining, mechanical engineering and finishing.

An article that covers it

Ce qui fait échouer à un examen Lean Six Sigma

Frequently asked questions

What are the 5M of the Ishikawa diagram?

Manpower, material, machine, method and mother nature — the environment. The sixth commonly added is measurement, and it deserves to be: a deviation may exist only in the instrument observing it. Other families circulate depending on the sector — this list is a memory aid meant to avoid blind spots, not an imposed nomenclature.

What is the difference between the Ishikawa diagram and the five whys?

The first sweeps across the width: it lists the families of possible causes so that one does not stop at the first idea that comes along. The second digs in depth along a single lead, going back from a symptom to its mechanism. They are used one after the other, in that order — list, then deepen the branch retained.

Does an Ishikawa diagram demonstrate the cause of a problem?

No, and that is the most frequent misreading about it. A cause written on a bone is a hypothesis formulated by a group, nothing more. Data is then needed — a count, a comparison, a trial — to decide between the branches. Without that step, what has been produced is a meeting, not an analysis.

Standardised term

ISO 13053-2 — tools and techniques

The authoritative definition is in that standard. This entry is an explanation written by us: it does not reproduce the text and does not replace it.