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Introduction to the Series

The Seven QC Tools —— check sheets, stratification, Pareto charts, cause-and-effect diagrams (fishbone diagrams), scatter diagrams, histograms, and control charts —— are the most fundamental and powerful "seven weapons" in quality management.

Many QC personnel use them, but not all truly understand them; many quality engineers know what they are, but do not fully utilize them. The purpose of this series is to dissect each tool: what it is for, when to use it, how to use it correctly, common pitfalls in practical application, and how to progress from "using it" to "mastering it."

This issue starts with the fishbone diagram. It is the most widely used tool among the Seven QC Tools, but also the one most easily misused.

Chapter 1: The Essence of Fishbone Diagrams

1.1 What is a Fishbone Diagram?

The fishbone diagram (Fishbone Diagram), also known as the cause-and-effect diagram (Cause-and-Effect Diagram), was first introduced by Japanese quality management master Kaoru Ishikawa in 1943, hence it is often referred to as the Ishikawa Diagram.

Standard Definition: The fishbone diagram is a visual tool that uses systematic causal chain analysis to unfold potential causes of a problem by category. Its core logic is: any result (problem/effect) has multiple causes, and these causes have hierarchical and categorical relationships.

1.2 The Essence of Fishbone Diagrams is Not "Drawing," but "Thinking Framework"

Many beginners view the fishbone diagram as "drawing a fish on the wall in a meeting room" — this misses its value entirely.

The essence of the fishbone diagram:
  A structured, systematic, visual framework for cause analysis.

  → Structured: Not random, but categorized according to 5M1E
  → Systematic: Covers all possible dimensions of causes, avoiding omissions
  → Visual: A single diagram to see all potential causes and their relationships

1.3 Three Major Functions of Fishbone Diagrams

Chapter 2: Structure and Elements of Fishbone Diagrams

2.1 Standard Structure Diagram

                  Cause Category 1                Cause Category 2
                       │                       │
       ┌───────────────┼───────────────────────┤
       │               │                       │
       ▼               ▼                       ▼
  ┌────────┐    ┌───────────┐          ┌───────────┐
  │ Sub-Cause│    │  Sub-Cause  │          │  Sub-Cause  │
  │  ├Main Cause│    │   ├Main Cause  │          │   ├Main Cause  │
  │  │ ├Major Category│    │   │ ├Major Category  │          │   │ ├Major Category  │
  └────────┘    └───────────┘          └───────────┘
       │               │                       │
       └───────────────┼───────────────────────┘
                       │
                       ▼
              ┌──────────────────┐
              │   Problem/Effect  │
              │  (Fish Head - Right)  │
              └──────────────────┘
       ┌───────────────┼───────────────────────┐
       │               │                       │
       ▼               ▼                       ▼
  ┌────────┐    ┌───────────┐          ┌───────────┐
  │ Cause  │    │  Cause    │          │  Cause    │
  │ Category 3  │  Category 4  │          │  Category 5  │
  └────────┘    └───────────┘          └───────────┘

2.2 Core Elements of Fishbone Diagrams

2.3 Types of Fishbone Diagrams (by Direction)

Chapter 3: 5M1E —— The "Skeleton" of Fishbone Diagrams

3.1 Complete Meaning of 5M1E

5M1E = Man (人) + Machine (机) + Material (料) + Method (法) + Measurement (测) + Environment (环)

This is the "standard classification framework" for fishbone diagram analysis in manufacturing.

Below are typical cause paths for each dimension:

M1 —— Man (人 / Personnel)

In-depth Comment: "Human" issues are often attributed to "bad attitude" or "lack of responsibility" — this is a major pitfall in analysis. Excellent quality professionals will continue to ask: Why is the attitude bad? Is it due to inadequate training? Insufficient incentives? Or is the process design itself prone to errors (the necessity of poka-yoke)?

M2 —— Machine (机 / Equipment)

M3 —— Material (料 / Materials)

M4 —— Method (法 / Methods)

M5 —— Measurement (测 / Measurement)

In-depth Comment: Issues with the measurement system itself are often overlooked — many people assume the measurement results are correct. Excellent quality engineers always ask: Is this data reliable? Is there a problem with the measurement system?

M6 —— Environment (环 / Environment)

3.2 5M1E Extensions in Different Industries

Chapter 4: Operating Procedures for Fishbone Diagrams (Standard 7-Step Method)

Step 1 —— Define the Problem (Fish Head)

Key: The more specific the problem definition, the more effective the analysis.

× Incorrect Example: "Poor product quality"
  → Too vague, difficult to focus the analysis direction

✓ Correct Example: "The welding strength of product B on line A has decreased by 30% over the past two weeks compared to industry standards"
  → Specific to: what product, what process, what metric, what time, what deviation amount

Problem Definition Checklist:

• Is the problem measurable? (Specific data/metrics)
• Is the problem time and space-limited? (What product, what time, what line)
• Are known factors excluded? (Are certain conditions known to be unchanged?)
• Is the problem a single focus? (Analyze one problem at a time)

Step 2 —— Build the Framework (Draw the Main Line and Major Bones)

Draw the basic structure of the fishbone diagram on paper or a whiteboard:

• Draw a horizontal thick line (main line)
• Draw a box on the right (fish head) and write the problem description
• Draw 5-8 diagonal arrows (major bones) and label them with cause categories (5M1E)

Step 3 —— Brainstorm (Team Collaboration)

Invite a cross-functional team to participate — the greatest value of the fishbone diagram lies in the collision of multiple perspectives.

Suggested Participants:
  └── QC (Most familiar with on-site details)
  └── QE (Most familiar with quality data and system issues)
  └── Production Operators/Team Leaders (Most familiar with actual operations)
  └── Process Engineers (Most familiar with process parameters)
  └── Equipment Maintenance Personnel (Most familiar with equipment status)

Brainstorming Rules:

• Do not criticize or judge any opinions (no matter how "absurd")
• Encourage quantity; more ideas can lead to better solutions
• Encourage "piggybacking" — building on others' ideas
• Describe each cause in neutral language (do not presuppose conclusions)
• Record every idea — do not filter

Step 4 —— Layered Expansion (Small Bones → Fine Bones)

Key Principle: Ask at least three levels deep.

Example (Increased defect rate → Welding defects):

First Level (Major Bone → Medium Bone):
  M (Method): Unreasonable welding parameters

Second Level (Medium Bone → Small Bone):
  Unreasonable welding parameters → Deviation in welding current settings

Third Level (Small Bone → Fine Bone):
  Deviation in welding current settings → Parameters not adjusted to standard after shift change
  Deviation in welding current settings → Failure of the equipment's parameter memory function

Fourth Level (Fine Bone → More Detailed):
  Failure of the equipment's parameter memory function → Preventive maintenance not covering this function module
  Failure of the equipment's parameter memory function → Parameters not verified after repairs

In this example, the initial symptom is "welding defects,"
the surface cause is "parameter deviation,"
and the deep cause is "incomplete maintenance system."

Excellent fishbone diagrams find the true root cause at the 3rd to 4th level. Stopping at the 1st to 2nd level results in superficial analysis of surface causes.

Step 5 —— Identify Key Influencing Factors

There are many causes on a fishbone diagram (usually 30-50), and not all need to be addressed.

Screening Methods:

Method 1: Voting (Team Consensus)
  Each person selects the top 5 most important causes from all causes
  Rank by the number of votes, select the top 5-8 causes
  Applicable: When the team is experienced

Method 2: Data Verification
  Use existing inspection data to verify the "relevance" of each cause
  For example: If it is "cause of equipment A," compare defect rates when equipment A is running and not running
  Applicable: When data is available

Method 3: Pareto Chart Screening
  Convert the causes identified by the fishbone diagram into data collection
  Use a Pareto chart to identify the "vital few" causes
  Applicable: When strict quantification is needed

Method 4: Control Experiment
  Conduct a small-scale intervention experiment on suspected root causes
  If the problem disappears after the intervention, the root cause is found
  Applicable: In controllable scenarios

Step 6 —— Verify Root Causes

Important Understanding: The causes identified by the fishbone diagram are just "hypotheses," not "conclusions."

Fishbone Diagram → Generate Hypotheses (Hypothesis Generation)
Data Verification → Verify Hypotheses (Hypothesis Verification)

A fishbone diagram without data verification is just a "pretty picture."

Three Levels of Root Cause Verification:

Step 7 —— Develop Improvement Actions

For verified root causes, develop specific improvement actions:

Root Cause: Failure of the equipment's parameter memory function (not covered by preventive maintenance)

Actions:
  Short-term: Immediately repair the parameter memory module, verify parameters after line stop for maintenance
  Medium-term: Include the parameter memory function in the preventive maintenance checklist
  Long-term: Increase reliability requirements for the parameter memory function in equipment procurement

Chapter 5: Common Pitfalls and Avoidance in Fishbone Diagrams

Pitfall 1: Listing Causes Without Categorization

× Incorrect Practice:
  List all causes directly on the fishbone diagram without categorization
  → Result: A mess of arrows pointing to the fishbone, no clear structure

✓ Correct Practice:
  Categorize using 5M1E before filling in
  → "Categorization itself is analysis" — ask "Are there any causes in this category?" for each category

Pitfall 2: Only Writing Major Bones, No Further Subdivision

× Incorrect Practice:
  Write "Man" on the major bone and do not expand further
  → Result: Too little information, no analytical value

✓ Correct Practice:
  Expand each major bone to at least the 2nd-3rd level
  "Man" → "Insufficient Skills" → "Inadequate Training" → "Insufficient New Employee Training Period"

Pitfall 3: Stopping at "Surface Causes" on One Fishbone

× Incorrect Fishbone Analysis:
  "Increased defect rate"
  → Man: Unfamiliar operations
  → Machine: Equipment aging
  → Material: Poor quality
  Then stop.
  → Result: Unable to develop effective countermeasures

✓ Correct Fishbone Analysis:
  Continue asking until you reach a level where "something can be done to change it"

Pitfall 4: One Person Completes the Entire Diagram

× Incorrect Practice:
  A quality engineer completes the fishbone diagram alone in the office
  → Result: A "expert perspective" diagram that misses real production line information

✓ Correct Practice:
  Cross-functional team (QC + Production + Process + Equipment + ...) works together
  → Different perspectives complement each other, uncovering hidden causes

Pitfall 5: Treating the Fishbone Diagram as a "Closing Report"

× Incorrect Practice:
  A beautiful fishbone diagram in an 8D report, then... no one looks at it again
  → Result: A pretty PPT, but the problem will reoccur

✓ Correct Practice:
  The fishbone diagram is an analysis tool, not a presentation tool
  The key is whether the conclusions from the analysis have been converted into improvement actions

Pitfall 6: Confusing "Causes" and "Phenomena"

× Incorrect: "High defect rate" is caused by "dimensional tolerance exceeded"
  → Dimensional tolerance exceeded is itself a nonconformity, not a cause

✓ Correct:
  Nonconformity: Dimensional tolerance exceeded
  Cause: Tool wear → Unreasonable tool life setting → Lack of tool management system

Pitfall 7: Attempting to Analyze Multiple Problems on One Fishbone Diagram

× Incorrect Practice:
  One fishbone diagram lists "increased scrap rate, increased customer complaints, decreased efficiency"
  → Result: Multiple problems mixed, causes intersect, no clear starting point

✓ Correct Practice:
  Analyze one problem at a time
  If there are multiple problems, draw separate fishbone diagrams

Chapter 6: Combining Fishbone Diagrams with Other Tools

6.1 Fishbone Diagram + 5 Whys

Best Partner. The fishbone diagram provides "breadth," while 5 Whys provides "depth."

Combined Use Process:

Step 1: Fishbone Diagram Brainstorming
  → List all possible causes (broad coverage)

Step 2: Use voting/data to select key causes
  → Identify the top 3-5 most likely causes

Step 3: Apply 5 Whys to each key cause
  → Drill down to the system-level root cause (deep analysis)

6.2 Fishbone Diagram + Pareto Chart

Combined Use:

Step 1: Fishbone Diagram Analysis → List all cause hypotheses
Step 2: Design data collection for the cause hypotheses
Step 3: Analyze the collected data using a Pareto chart
  → Identify the "vital few" causes
Step 4: Develop improvement actions for the key few causes

This method addresses the inherent weakness of the fishbone diagram: The fishbone diagram generates many hypotheses but does not prioritize them, while the Pareto chart adds priority judgment.

6.3 Fishbone Diagram + FMEA

Combined Use:

Step 1: Use the fishbone diagram to identify potential failure causes in the process
Step 2: Input the causes identified by the fishbone diagram into the "failure cause" column of the FMEA
Step 3: Evaluate the severity (S), occurrence (O), and detection (D) of the cause
Step 4: Calculate the RPN to determine improvement priorities

FMEA's O (occurrence) and D (detection) judgments require fishbone diagram information as input.

6.4 Fishbone Diagram + C&E Matrix (Cause & Effect Matrix)

The C&E matrix is a "quantitative upgrade" of the fishbone diagram:

Approach:
  ├── Rows: All causes identified by the fishbone diagram
  ├── Columns: Product quality characteristics (CTQs)
  ├── Cells: Impact scores of causes on CTQs (0, 1, 3, 9)
  └── Summary: Total score for each cause → Rank the most critical causes

Chapter 7: Practical Cases of Fishbone Diagrams in Various Industries

Case 1: Manufacturing —— "Insufficient Welding Strength"

Problem: The welding strength nonconformity rate of product B on line A increased from 0.3% to 2.5%

Fishbone Diagram Analysis Path (Simplified):