Fishbone diagram & 5 Whys
Root cause analysis in the Analyze phase has two key tools: the fishbone diagram (also called Ishikawa or cause-and-effect diagram) for structured brainstorming of potential causes, and 5 Whys for drilling down from a symptom to its underlying root cause. Neither tool proves causation — that requires data. They generate hypotheses to test.
The fishbone diagram: 6M categories
The effect (problem) is written at the head of the fish. Six major cause categories form the bones:
| Category | What goes here | Example for fill weight issue |
|---|---|---|
| Man (People) | Operator skill, training, attention, fatigue | Operator not following fill weight check frequency; variable technique |
| Machine (Equipment) | Gage accuracy, maintenance, wear, settings | Fill nozzle wear; scale calibration drift; auger speed variation |
| Method (Process) | Procedure, work instruction, test method | SOP doesn’t specify warm-up checks; no defined tare procedure |
| Material | Raw materials, components, supplier variation | Bulk density variation in Examplorphine API lot; bottle weight variation |
| Measurement | Gage R&R, calibration, measurement method | High %GRR on in-process scale; wrong measurement point in process |
| Mother Nature (Environment) | Temperature, humidity, vibration, dust | Temperature variation affects tablet flow rate; vibration from nearby equipment |
How to run a fishbone session
- Write the problem statement on the right (the "head"). Be specific: "Fill weight on Line 2 exceeds 255 mg, occurring on ~8% of bottles."
- Draw six major bones and label with the 6M categories.
- Brainstorm causes for each category without judgment. Each cause is a "rib" off the main bone; sub-causes are smaller ribs.
- Look for causes that appear in multiple categories — they are worth a closer look.
- Circle or dot-vote the top 3–5 most likely causes to investigate with data.
5 Whys technique
Start with the problem statement and ask "Why?" repeatedly until you reach a root cause that, if fixed, would prevent recurrence. Five iterations is a guideline, not a rule.
Problem: Fill weight on Line 2 averages 251.5 mg, target 250.0 mg.
Why 1: Why is the fill weight high?
Because the auger is dispensing more powder than set.
Why 2: Why is the auger dispensing more than set?
Because the auger speed control module was replaced last month and re-calibrated to an old spec.
Why 3: Why was it calibrated to an old spec?
Because the maintenance team used a paper SOP, not the current electronic version.
Why 4: Why was the current electronic version not used?
Because maintenance technicians have been printing SOPs and there is no control preventing use of outdated paper copies.
Root cause: The document control system does not prevent use of obsolete paper SOP copies for maintenance tasks.
Corrective action: Move maintenance calibration SOPs to electronic-only; add a step in the maintenance work order requiring the technician to confirm the current revision number before performing work.
5 Whys best practices
- Answer each "Why" with a fact, not an assumption. If you don’t know, collect data before continuing.
- Multiple causes may branch. One symptom can have several independent root causes. Draw separate 5 Whys chains for each branch.
- Stop when the root cause is actionable. If the answer to "Why?" is "because of gravity" or "because humans make errors," you’ve gone too far. Stop at a cause the team can actually control.
- Validate root causes with data. The Analyze tollgate should be based on evidence, not just a completed fishbone. Use Pareto charts, scatter plots, or hypothesis tests to confirm the top suspects.
Common exam traps
- Fishbone is brainstorming, not proof. The diagram identifies hypotheses. Data confirms or rules them out.
- 5 Whys can lead to the wrong root cause if any answer is wrong. Validate each "Why" response before continuing.
- Asking "5 Whys" is not the same as asking the same question five times. Each iteration answers the previous one and goes deeper.
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