DMAIC|5 min read
Fishbone (Ishikawa) diagram: the 6M method, with a real example
The fishbone — also called the Ishikawa or cause-and-effect diagram — organizes possible causes of a problem across 6 categories: Manpower, Machine, Method, Material, Measurement, Environment. It's a brainstorming scaffold, not an answer.
The 6Ms
| Category | What it covers | Example (vial reject) |
|---|---|---|
| Manpower | People, skills, training, fatigue | B-shift operator training gap on changeover |
| Machine | Equipment, tools, wear, calibration | Torque driver drift after 4 hours |
| Method | Procedures, sequence, SOP gaps | Changeover SOP missing torque verification step |
| Material | Inputs, supplier variation, lots | Cap supplier lot variation (2 of 6 out-of-spec) |
| Measurement | Gages, sampling, inspection | Visual inspection criteria subjective |
| Environment | Temperature, humidity, layout | HVAC cycling at 02:00 causes seal pressure drift |
How to run it
- Start with the problem (effect) on the right side
- Draw the 6 category 'bones'
- Brainstorm possible causes — broad, not filtered
- Dig one level deeper on each cause with a 'why?' — that's where root causes live
- Mark the 3-5 most likely candidates for testing in the Analyze phase
The fishbone trap
Most teams build a beautiful fishbone and then never test a single cause. The fishbone is the start of analysis, not the end. Every candidate must be tested with process data before it earns 'confirmed root cause' status.
When to use 5-Why instead
If the cause is clearly in one category, skip the fishbone and go straight to a 5-Why chain. Fishbone shines when the problem could live anywhere and you need a scaffold to explore broadly first.