Lean Six Sigma tools

When to use a fishbone diagram (and when not to)

A fishbone diagram (also called an Ishikawa diagram) is the right tool when you need to systematically identify all possible causes of a specific problem before validating them with data. It is not the right tool when you already know the root cause or when the problem is simple enough that a 5 Whys analysis would suffice.

Five situations where a fishbone diagram is the best choice

The problem has multiple possible causes and you need to brainstorm systematically before narrowing down
A cross-functional team needs to contribute perspectives — the structured categories ensure everyone is heard
You are in the Analyse phase of a DMAIC project and need to generate hypotheses for statistical testing
The problem is recurring and previous quick fixes have not worked — a structured cause analysis is needed
You need a visual artefact for a team workshop that can be displayed, photographed, and referenced later

Three situations where a different tool works better

You already know the root cause: skip the fishbone and go straight to a 5 Whys or direct action plan. A fishbone when you already know the answer wastes team time.
The problem has a single obvious cause: use a 5 Whys analysis instead. It is faster and more focused for simple, linear cause-and-effect chains.
You need to prioritise causes by frequency or impact: use a Pareto chart after the fishbone. The fishbone identifies causes; the Pareto chart tells you which ones matter most.

How to run a fishbone session in 4 steps

Write the problem statement at the head of the fishbone (right side). Be specific: “30% of outpatient appointments overrun by 15+ minutes” is better than “waiting times are too long.”
Draw the 6M categories as branches: Man (people), Machine (equipment), Method (process), Measurement (data), Material (inputs), and Environment (surroundings). Ask the team to call out possible causes for each branch.
Capture every cause without filtering or debating. The goal is breadth, not consensus. Use sticky notes or a whiteboard so causes can be moved or grouped later.
Prioritise and validate the top 3 to 5 causes with dot voting, then plan data collection to confirm or eliminate each one before taking action.

Download the Fishbone Diagram template

SimplicityHub's free Fishbone Diagram template includes a completed example showing how the 6M framework is applied to a real improvement project.

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Frequently asked questions

What are the 6Ms in a fishbone diagram?

The 6Ms are the six categories used to structure cause analysis: Man (people skills, training, staffing), Machine (equipment, tools, technology), Method (processes, procedures, workflows), Measurement (data quality, metrics, inspection), Material (inputs, supplies, raw materials), and Environment (workspace, conditions, external factors). Some teams add a seventh category, Management, for organisational or policy-related causes.

Is a fishbone diagram the same as an Ishikawa diagram?

Yes. The fishbone diagram is also known as an Ishikawa diagram (after its creator, Kaoru Ishikawa) or a cause-and-effect diagram. All three terms refer to the same tool. “Fishbone” refers to the visual shape; “Ishikawa” to the creator; “cause-and-effect” to the function.

How many causes should a fishbone diagram have?

There is no fixed number, but a well-run session typically generates 15 to 30 potential causes across the 6M categories. The goal is not to list every conceivable cause but to ensure the team has explored each category thoroughly. After the session, prioritise 3 to 5 causes for data validation rather than attempting to investigate all of them.