Introduction to the Series

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 these tools, but not everyone truly "understands" them; many quality engineers know what they are, but do not "master" their application. The purpose of this series is to dissect each tool: what it does, when to use it, how to use it correctly, common pitfalls in practical application, and how to move from "using" to "mastering" it.

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

Chapter 1: The Essence of the Fishbone Diagram

1.1 What is a Fishbone Diagram

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

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

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

Many beginners treat 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, and visual framework for cause analysis.

  → Structured: not random writing, 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 the Fishbone Diagram

Chapter 2: Structure and Elements of the Fishbone Diagram

2.1 Standard Structure Diagram

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

2.2 Core Elements Comparison

2.3 Types of Fishbone Diagrams (By Direction)

Chapter 3: 5M1E —— The "Skeleton" of the Fishbone Diagram

3.1 The Full Meaning of 5M1E

5M1E = Man (Personnel) + Machine (Equipment) + Material (Materials) + Method (Process) + Measurement (Measurement) + Environment (Environment)

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

Below are typical cause paths for each dimension:

M1 —— Man (Personnel)

Deep Insight: "Personnel" 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 it because the process design itself makes it easy for people to make mistakes (the necessity of poka-yoke)?

M2 —— Machine (Equipment)

M3 —— Material (Materials)

M4 —— Method (Process)

M5 —— Measurement (Measurement)

Deep Insight: Issues with the measurement system itself are often overlooked — many people assume the measurement results are correct. Excellent QEs always ask: Is this data reliable? Is there a problem with the measurement system?

M6 —— Environment (Environment)

3.2 5M1E Extensions in Different Industries

Chapter 4: The Fishbone Diagram Operation Process (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, from industry standards"
  → Specific to: what product, what process, what metric, what time, what deviation

Problem Definition Checklist:

• Is the problem measurable? (Specific data/metrics)
• Is the problem time and location specific? (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 side (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 assume conclusions)
• Record every idea — do not filter

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

Key Principle: Ask at least three layers of questions.

Example (Increase in defect rate → Welding defects):

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

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

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

Fourth Layer (Fine Bone → Further Detailed):
  Failure of the parameter memory function → Preventive maintenance did not cover this function module
  Failure of the 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."

An excellent fishbone diagram finds the true root cause at the 3rd-4th layer. If the analysis stops at the 1st-2nd layer, it only provides a superficial analysis of "surface causes."

Step 5 —— Identify Key Influencing Factors

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

Screening Methods:

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

Method 2: Data Verification
  Use existing inspection data to verify the "relevance" of each cause
  For example: If the cause is "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 in the fishbone diagram into data collection
  Use a Pareto chart to analyze the data
  → Identify the "vital few" causes
  Applicable: When strict quantification is required

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 in the fishbone diagram are just "hypotheses," not "conclusions."

Fishbone Diagram → Hypothesis Generation
Data Verification → Hypothesis Verification

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

Three Levels of Root Cause Verification:

Step 7 —— Develop Improvement Actions

Based on the verified root causes, develop specific improvement actions:

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

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 Approach:
  List all thought-of causes directly on the fishbone diagram without categorization
  → Result: A bunch of arrows pointing to the fishbone, chaotic and unorganized

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

Pitfall 2: Writing Only Major Bones, No Subdivision

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

✓ Correct Approach:
  Each major bone should be expanded to at least the 2nd-3rd layer
  "Man" → "Insufficient Skills" → "Inadequate Training" → "Insufficient Training Period for New Employees"

Pitfall 3: Stopping at "Surface Causes"

× Incorrect Fishbone Analysis:
  "Increase in defect rate"
  → Man: Inexperienced operation
  → Machine: Aging equipment
  → Material: Poor quality material
  Then it stops.
  → Result: Unable to develop effective countermeasures

✓ Correct Fishbone Analysis:
  Continue asking until "something can be done to change it"

Pitfall 4: One Person Drawing the Entire Diagram

× Incorrect Approach:
  A quality engineer draws the fishbone diagram alone in the office
  → Result: A "expert perspective" diagram, missing real production line information

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

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

× Incorrect Approach:
  Draw a beautiful fishbone diagram in an 8D report, then... no one looks at it again
  → Result: A pretty PPT, but the problem will reappear next time

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

Pitfall 6: Confusing "Causes" and "Phenomena"

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

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

Pitfall 7: Attempting to Analyze Multiple Problems with One Diagram

× Incorrect Approach:
  List "increase in scrap rate, increase in customer complaints, decrease in efficiency" on one fishbone diagram
  → Result: Multiple problems mixed together, causes intersect, no clear starting point

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

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 most likely 3-5 causes

Step 3: Apply 5 Whys to each key cause
  → Dig 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 based on cause hypotheses
Step 3: Analyze the collected data using a Pareto chart
  → Identify the "vital few" causes
Step 4: Develop improvement actions for the vital few causes

This method addresses the inherent weakness of the fishbone diagram: The fishbone diagram generates many hypotheses but cannot prioritize them. The Pareto chart supplements the prioritization.

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 in the fishbone diagram into the "failure cause" column of the FMEA
Step 3: Evaluate the severity (S), occurrence (O), and detection (D) of each cause
Step 4: Calculate the RPN to determine the improvement priority

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 in 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 nonconformity rate of welding strength for product B on line A increased from 0.3% to 2.5%

Fishbone Diagram Analysis Path (Simplified):

Knowledge code: 5.2.4

Author: QTank