Fishbone Diagram
The fishbone (Ishikawa) diagram is a visual root cause analysis tool. Learn how to create one, the categories to consider, and when to use it.
What is a Fishbone Diagram?
A fishbone diagram (also called an Ishikawa diagram or cause-and-effect diagram) is a visual tool for identifying and organising the possible causes of a problem. The problem statement is at the 'head' of the fish, and major cause categories branch off the 'spine'. Specific causes are added as 'bones' on each branch.
It is especially useful for complex problems where multiple factors may contribute, and for group brainstorming sessions where a team identifies causes together.
The 6M Categories
The traditional fishbone diagram uses six categories for causes, known as the 6Ms:
- Man (People): Training, skills, experience, fatigue, motivation, communication.
- Machine (Equipment): Equipment condition, age, maintenance history, capacity, calibration.
- Method (Process): Procedures, work instructions, process design, workflow.
- Material: Raw materials, spare parts, consumables, specifications, quality.
- Measurement: Sensors, gauges, calibration, inspection methods, data accuracy.
- Environment: Temperature, humidity, dust, vibration, lighting, space constraints.
How to Create a Fishbone Diagram
- State the problem clearly. Write it in the 'head' of the fish (right side).
- Draw the spine (horizontal line) pointing to the head.
- Add the six category branches at an angle to the spine.
- Brainstorm possible causes for each category. Write each cause as a sub-branch.
- For each cause, ask 'why does this happen?' and add sub-causes as smaller branches.
- Review the completed diagram. Identify the most likely causes.
- Investigate the most likely causes. Collect data to confirm or eliminate them.
- Implement corrective action on the confirmed root cause(s).
When to Use a Fishbone Diagram
- Complex problems: When the problem has multiple possible causes and you need to organise them systematically.
- Team analysis: When you want to involve a team in identifying causes. The visual format encourages participation.
- Recurring problems: When the same problem keeps happening despite previous fixes. The fishbone helps identify causes that were missed.
- New problems: When you don't know where to start investigating. The 6M categories prompt you to consider all angles.
Fishbone vs Five Whys
The Five Whys is faster and simpler for straightforward problems with a single cause chain. The fishbone is better for complex problems with multiple potential causes. Use Five Whys for quick investigations; use fishbone for major incidents or recurring problems that warrant a team analysis.
Tip: After completing the fishbone diagram, prioritise the causes. Not all causes are equally likely or impactful. Use a simple scoring: high/medium/low likelihood and high/medium/low impact. Focus investigation on the high-likelihood, high-impact causes first. Don't try to fix everything at once.