Fishbone diagram (Ishikawa / cause-and-effect)
A fishbone diagram—also called an Ishikawa diagram or cause-and-effect diagram—is a visual brainstorming tool that organizes potential causes of a problem into categories branching off a central “fish spine.” The problem statement goes in the fish’s head (the effect); causes fan out to the left.
It does not identify root causes by itself. It captures and organizes the team’s hypotheses so they can be tested. The fishbone and the 5 Whys are often used together: the fishbone generates candidate causes; the 5 Whys drills into each branch.
The 6M categories (manufacturing)
| Category | Includes |
|---|---|
| Man (People) | Operator training, fatigue, technique, communication |
| Machine | Equipment condition, calibration, maintenance, settings |
| Material | Raw material variation, incoming quality, storage |
| Method | SOP accuracy, work instruction clarity, process sequence |
| Measurement | Gage accuracy, gage R&R, inspection technique |
| Mother Nature (Environment) | Temperature, humidity, vibration, lighting |
Service processes often use the 5P (or 4S) variant: People, Process, Policy, Place, Product. Choose the categories that fit your context.
How to build a fishbone diagram
- Write the effect (the problem) in a box at the right. Draw a horizontal arrow pointing to it.
- Draw diagonal branches for each major category (the 6 Ms).
- Brainstorm causes under each category. Each cause gets a smaller branch.
- Ask “Why?” on each cause to add sub-branches (linking to the 5 Whys technique).
- Circle the most likely root causes for further data collection and verification.
Worked example
Effect: Filling machine produces capsules outside weight specification.
- Machine: auger speed drift after warm-up; worn seals; vibration from adjacent press
- Method: tare not zeroed at shift start; SOP does not specify warm-up time
- Material: bulk density variation lot-to-lot; clumping when humidity >65%
- Man: different operators use different start-up procedures
- Measurement: checkweigher last calibrated 14 months ago; sample frequency too low
- Mother Nature: HVAC cycles cause RH fluctuation on the production floor
The team circles “bulk density variation” and “auger speed drift” as the two most likely contributors and runs a designed experiment to confirm.
Common exam traps
- The fishbone is a hypothesis generator, not a root-cause finder. Causes on the diagram are guesses until data confirm them.
- Every branch should be a cause, not a symptom. “High defect rate” is the effect. “Operators untrained” is a cause.
- Do not skip the measurement branch. A surprisingly large fraction of quality problems trace back to the measurement system, not the process itself.
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