Ishikawa diagram (fishbone diagram) with example

An Ishikawa diagram, also called a fishbone diagram or cause-and-effect diagram, is a drawing that organises all possible causes of one problem. The problem sits in the head of the fish, and the causes hang off the spine like bones, grouped into fixed categories such as the 6Ms. You use it so that, when a nonconformity occurs, you do not stop at the first explanation you come across.

The method was devised by the Japanese quality expert Kaoru Ishikawa. It is described as a technique in the international standard IEC 31010:2019 on risk assessment techniques. Within ISO certification, you mainly use the fishbone diagram for nonconformities and corrective actions in ISO 9001 and for CAPA in ISO 13485. It is one of the ways to carry out the root cause analysis that every standard requires.

Where does an ISO standard ask for an Ishikawa diagram?

No ISO standard prescribes the Ishikawa diagram word for word. However, all management system standards require you to determine the cause when a nonconformity occurs, in clause 10.2. How you do that is up to you.

Standard	Clause	What the standard requires
ISO 9001	10.2	Determine the cause of the nonconformity before choosing an action
ISO 14001 / ISO 45001	10.2	Same, for environmental nonconformities and incidents
ISO 27001	10.2	Same, for nonconformities in the ISMS
ISO 13485	8.5.2	Determine the cause of nonconformities within the CAPA process

Auditors like to see the fishbone diagram for recurring or complex problems. It shows that you searched widely and did not simply write down “human error”.

The 6Ms of the fishbone diagram

The classic breakdown has six categories. Each name starts with an M, in Dutch as in English: man (mens), machine (machine), method (methode), material (materiaal), measurement (meting) and environment (milieu).

Category	Question you ask	Examples of causes
Man	Is it down to knowledge, experience or staffing?	New employee, no instruction, time pressure
Machine	Is it down to equipment or software?	Wear, fault, wrong setting
Method	Is it down to the way of working or the procedure?	Procedure unclear, step missing
Material	Is it down to raw materials or parts?	Deviating delivery, wrong specification
Measurement	Is it down to how you measure or inspect?	Measuring instrument not calibrated, no final inspection
Environment	Is it down to the conditions?	Temperature, humidity, noise, workplace

The categories are a tool, not a rule. Ishikawa himself encouraged adapting them. A service provider often uses: people, processes, systems, information, suppliers and customers.

Example: Ishikawa diagram for a complaint about leaking installations

An installation company receives seven complaints within three months about leaking central heating connections shortly after handover. During the internal audit, this becomes a nonconformity. The team puts the problem in the head of the fish and fills in the bones.


 Man                  Method                Material
  \ new fitters        \ no torque setting    \ new supplier
   \ no buddy           \ no leak test on      \ couplings
    \                    \ work order           \ wrong size
     \____________________\_____________________\________
                                                         > 7 leaks within 4 weeks
     /                    /                     /    after handover
    / tools              / no pressure test at / cold building site
   / torque wrench       / handover            / in winter
  / missing from van    /                     /
 Machine              Measurement           Environment

After that, the team assesses each cause: is there evidence that it really contributes? This is the step that many people skip.

Category	Possible cause	How it was checked	Confirmed?
Man	New fitters without supervision	5 of the 7 leaks were at fitters who had been employed for less than six months	Yes
Method	Work order does not require a pressure test	Work order reviewed: step missing	Yes
Material	Couplings from new supplier	Leaks also occur with the old couplings	No
Machine	Torque wrench missing from vans	3 of 8 vans without a torque wrench	Yes, contributing
Measurement	No pressure test at handover	Consequence of the work order, same cause	Yes
Environment	Cold on the building site	Leaks spread across the seasons	No

Conclusion: the main cause is a working method without a mandatory pressure test, reinforced by inexperienced fitters without a buddy. The team then digs deeper with the 5 whys method on the confirmed bones.

A fishbone diagram gives possible causes. You only have a cause once it has been checked against facts. So always put a “confirmed” column next to your diagram. That is exactly what an auditor wants to see.

How to make an Ishikawa diagram step by step

Describe the problem concretely

Not “quality is poor”, but “7 leaks within 4 weeks of handover, January to March”. A measurable problem keeps the discussion focused.

Choose your categories

Start with the 6Ms. Adapt them if they do not fit your organisation.

Brainstorm per bone

Do this with the people who do the work. Write everything down and do not judge anything yet. Allow half an hour to an hour.

Check against facts

For each cause, look for evidence in records, complaints or measurements. Cross out what does not hold up.

Choose the action

Address the confirmed causes. Record the action, owner and deadline in your corrective action register.

Ishikawa diagram or 5 whys: which do you choose?

Both methods look for the cause, but they work differently. A fishbone goes wide, 5 whys goes deep. For complex problems, you combine them.

	Ishikawa diagram	5 whys
Direction	Wide: many possible causes	Deep: one chain of causes
Suitable for	Complex or recurring problems	Single, clear-cut nonconformities
Number of people	Team of 3 to 6	Can be done alone or with two
Risk	List without evidence	One track, other causes missed

For an extensive problem-solving process with a customer, such as in the automotive industry, the fishbone diagram often sits in step D4 of an 8D report.

Common mistakes with the fishbone diagram

“Human error” as the end point. Keep asking: why was that error possible? Was an instruction, a check or enough time missing?
Making the diagram alone. The quality manager thinks up the causes at their desk. The shop floor knows the real causes.
No check against facts. A full diagram looks thorough, but says nothing without evidence.
Problem too broad. “Customers dissatisfied” produces a hundred bones. Make the problem smaller.