Fishbone Analysis

Fishbone (Ishikawa) Analysis

When to use

Use fishbone analysis to brainstorm all possible causes of a quality problem before running 5-Why. It prevents tunnel vision and ensures no cause category is overlooked. Particularly valuable for complex defects with multiple potential contributing factors.

Typical use: 8D D4 brainstorming session, CAPA root cause investigation, initial problem analysis.

Prerequisites

• Problem clearly defined with Is/Is-Not or 5W2H
• Cross-functional team (quality, production, engineering at minimum)
• Access to the process, machine, or product where defect occurred

Required Fishbone Checklist

☐ Problem statement defined and agreed before starting — no cause language in the problem statement ☐ All 6M categories addressed — at least one entry per M, or documented justification if a category is not applicable ☐ Brainstorming completed before any evaluation or elimination — do not evaluate while generating ☐ Every cause classified as Confirmed, Probable, or Unlikely using objective evidence — not opinion alone ☐ Confirmed = supported by data or physical evidence; Probable = logical, consistent with Is/Is-Not, not yet confirmed; Unlikely = contradicted by data ☐ Confirmed and Probable causes cross-checked against existing PFMEA failure cause entries before proceeding to 5-Why ☐ Each Confirmed or Probable cause carries forward to its own 5-Why chain ☐ After root cause confirmed: horizontal deployment check — could the same cause exist in similar parts, processes, or product families?

The 6M Framework

The six main "bones" of the fish. All six must be addressed. If a category genuinely does not apply, document: "No causes identified in this category after structured team review — not applicable because [reason]."

1. Man (Human Factors)

Questions to ask:

• Was the task performed by a trained, qualified operator?
• Is competence documented and current?
• Was the operator following the correct work instruction?
• Could fatigue, shift change, or distraction contribute?
• Is the task ergonomically difficult or error-prone?
• Is the same defect found on all shifts or only one? (single-shift → operator-specific)

2. Machine (Equipment)

Questions to ask:

• Is the machine calibrated? When was it last calibrated? Is it within interval?
• Has preventive maintenance been performed on schedule?
• Are there any known deviations (vibration, wear, temperature drift)?
• What is the machine capability (Cp, Cpk)? Is it capable for this feature?
• Is tooling worn or damaged?
• Is the jig or fixture functioning correctly?

3. Method (Process)

Questions to ask:

• Is there a documented work instruction for this operation?
• Is the work instruction current and at the workstation?
• Are process parameters (temperature, pressure, speed, torque) specified and controlled?
• Is there a control plan entry for this process step?
• Is the process sequence correct?
• Is the method different between shifts or operators?

4. Material (Input Material)

Questions to ask:

• Does the defect correlate with a specific incoming batch or supplier lot?
• Has the material specification been met (certificate of conformance, incoming inspection)?
• Has the material been stored correctly (temperature, humidity, FIFO)?
• Is the material traceable to its origin?
• Has the supplier changed anything recently (sub-supplier, process, location)?

5. Measurement (Measurement System)

Questions to ask:

• Is the measurement system capable (MSA / Gauge R&R performed)?
• Is the gauge calibrated and within its calibration interval?
• Is the measurement method standardised (same fixture, same operator technique)?
• Could measurement error mask the defect (false acceptable)?
• Are measurement results repeatable between operators?

6. Mother Nature / Environment

Questions to ask:

• Does the defect correlate with ambient temperature or humidity?
• Are there vibration or contamination sources nearby?
• Is lighting adequate for visual inspection?
• Are there seasonal patterns?
• Does the cleanroom or ESD environment meet requirements?

Workflow

Step 1 — Draw the diagram

Write the problem (effect) at the head (right side). Draw the spine. Add six main bones labelled with the 6M categories.

Step 2 — Brainstorm with the team

For each M category: "What in [M] could cause [the problem]?"

Capture all ideas without judgment — quantity first, evaluation second. Do not evaluate or discard during brainstorming. Allocate 30–60 minutes minimum. Time pressure is the most common reason causes are missed.

Add each cause as a sub-bone to the relevant M category. Sub-bones can branch further (cause of a cause).

Step 3 — Evaluate and prioritise

Mark each cause as:

• Confirmed (supported by data or direct physical evidence — not opinion)
• Probable (logical, consistent with the Is/Is-Not pattern, but not yet confirmed by data)
• Unlikely (contradicted by data or the Is/Is-Not pattern)

Discard Unlikely causes. Investigate Confirmed and Probable causes. A cause cannot be classified as Confirmed without objective evidence (measurement data, physical demonstration, reproduction test, or direct record review).

Step 4 — PFMEA cross-check

Before proceeding to 5-Why, cross-check Confirmed and Probable causes against the existing PFMEA:

• Is this failure cause already documented in the PFMEA? If yes, was its detection or prevention control supposed to prevent this defect?
• If the PFMEA did not capture this cause, flag it — the PFMEA must be updated after the root cause is confirmed.

Step 5 — Connect to 5-Why

For each Confirmed or Probable cause, run a 5-Why chain to reach the systemic root cause.

The fishbone identifies candidate causes. The 5-Why validates and deepens them.

After root cause is confirmed: assess whether the same cause could exist in similar parts, processes, or product families. Document horizontal deployment actions if applicable.

Output format

Document the fishbone as a table (easier to include in reports than a diagram):

All 6M categories must appear in the table. If a category has no entries after structured brainstorming, document: "No causes identified in this category after structured team review."

Common mistakes

• Brainstorming under time pressure — causes are missed; allocate 30–60 minutes minimum
• Stopping at first-level causes — "machine not calibrated" is a cause, but add sub-bone: why was it not calibrated?
• Not using data to confirm/discard — every cause must be validated against objective evidence, not just listed
• Using fishbone as the root cause — fishbone finds candidate causes; 5-Why finds root cause
• Skipping categories — if a category is left blank without justification, the analysis is incomplete and may miss the real cause
• Classifying causes as Confirmed based on team opinion — Confirmed requires evidence; without it, classify as Probable and plan verification

Output Format

At the start of each use, ask the user:

"How would you like to receive the output? A — Structured Markdown (formatted tables and sections, ready to copy) B — Plain tables (simplified structure for Excel or Word) C — Narrative report (flowing text for a formal document or email)

Default: A."

Adapt all output sections to the chosen format. If the platform or session context already defines a format preference, skip this question.

Changelog