Fishbone Diagram

Ishikawa Diagram / Cause-and-Effect Diagram

1. Overview of the Fishbone Diagram: A Causal Analysis Tool That Traces Root Causes Back from an Effect

    flowchart LR
    A["Removing only the immediate<br/>cause after a problem occurs<br/>results in recurrence"] --"6M classification system<br/>hierarchical cause analysis"--> B["Systematically surface<br/>all potential causes"] --"Identify root cause<br/>and establish corrective action"--> C["Prevent recurrence<br/>fundamental quality improvement"]

    style A fill:#FFEBEE,stroke:#D32F2F,color:#000
    style B fill:#E3F2FD,stroke:#1976D2,color:#000
    style C fill:#E8F5E9,stroke:#388E3C,color:#000

Definition: A causal diagram developed by Kaoru Ishikawa, a pioneer of Japanese quality management, that represents the effect (problem or defect) as a fish’s head and categories of potential causes as the bones, systematically identifying and analyzing the root causes of a problem through the 6M classification system.

Characteristics: (Visual structuring) Represents complex cause-and-effect relationships in an intuitive fishbone-shaped diagram. (Team brainstorming) Effective as a tool for exhaustively surfacing causes from diverse perspectives. (Linked with other quality tools) Combined with 5-Why analysis and Pareto charts, it serves as a core analysis tool for quality circles and Six Sigma DMAIC.

2. Core Structure of the Fishbone Diagram

a. The 6M Cause Classification System

    flowchart LR
    subgraph CAUSES["Causes"]
        direction TB
        M1["Man<br/>Skill level, training, carelessness,<br/>fatigue, lack of motivation"]
        M2["Machine<br/>Equipment aging, breakdown,<br/>precision, maintenance"]
        M3["Method<br/>Work procedures, standards,<br/>process design flaws"]
        M4["Material<br/>Raw material quality,<br/>component specs, supply"]
        M5["Measurement<br/>Inspection accuracy,<br/>instrument error, criteria"]
        M6["Mother Nature (Environment)<br/>Temperature, humidity, dust,<br/>noise, vibration"]
    end

    EFF["Effect<br/>Problem, defect,<br/>quality characteristic"]

    M1 --> EFF
    M2 --> EFF
    M3 --> EFF
    M4 --> EFF
    M5 --> EFF
    M6 --> EFF

    style M1 fill:#E3F2FD,stroke:#1976D2,color:#000
    style M2 fill:#F3E5F5,stroke:#7B1FA2,color:#000
    style M3 fill:#FFF3E0,stroke:#F57C00,color:#000
    style M4 fill:#FFEBEE,stroke:#D32F2F,color:#000
    style M5 fill:#E8F5E9,stroke:#388E3C,color:#000
    style M6 fill:#E0F2F1,stroke:#00796B,color:#000
    style EFF fill:#1E3A5F,stroke:#1E3A5F,color:#fff

Detail of the 6M Cause Categories

Category	Meaning	Key Cause Factors	Example in IT/Software
Man	People	Insufficient skill, inadequate training, reduced focus, non-compliance with procedures	Developer coding mistakes, lack of security awareness
Machine	Equipment/systems	Aging equipment, malfunction, reduced precision, inadequate maintenance	Degraded server performance, legacy system failures
Method	Method/procedure	Absence of standards, inefficient processes, incorrect work sequence	No code review performed, deployment procedure errors
Material	Materials/software	Defective raw materials, non-conforming specs, supplier quality	Use of vulnerable open-source libraries
Measurement	Measurement/testing	Instrument error, ambiguous measurement criteria, inadequate inspection methods	Insufficient test coverage, misconfigured thresholds
Mother Nature	Environment	Temperature, humidity, dust, noise, unstable network environment	Unstable infrastructure environment, third-party API failures

b. Root Cause Analysis (RCA) Procedure

    flowchart LR
    S1["1. Define the problem<br/>Clarify the effect<br/>with a measurable,<br/>specific statement"]
    S2["2. Form a team<br/>Cross-functional brainstorming<br/>Collect cause ideas<br/>by 6M category"]
    S3["3. Build the diagram<br/>Large, medium, and small bones<br/>hierarchical<br/>structuring of causes"]
    S4["4. Prioritize causes<br/>In-depth 5-Why analysis<br/>Identify root cause<br/>based on data and evidence"]
    S5["5. Corrective action<br/>Eliminate root cause<br/>Establish and execute countermeasures<br/>verify effectiveness"]

    S1 --> S2 --> S3 --> S4 --> S5
    S5 -->|"Reanalyze if recurring"| S1

    style S1 fill:#E3F2FD,stroke:#1976D2,color:#000
    style S2 fill:#F3E5F5,stroke:#7B1FA2,color:#000
    style S3 fill:#FFF3E0,stroke:#F57C00,color:#000
    style S4 fill:#FFEBEE,stroke:#D32F2F,color:#000
    style S5 fill:#E8F5E9,stroke:#388E3C,color:#000

Example of Root Cause Exploration Combined with 5-Why

Step	Question	Answer
Problem	Why did the system failure occur?	The database response was delayed
Why 1	Why was the DB response delayed?	A specific query performed a full table scan
Why 2	Why did the full-scan query run?	It queried on a column condition with no index
Why 3	Why was there no index?	There was no performance review procedure during development
Why 4	Why was there no performance review procedure?	The code review checklist did not include DB performance items
Root cause	→ Method category — missing DB performance item in the code review checklist

3. Expected Benefits and Application of the Fishbone Diagram

Category	Expected Benefits	Application and Practical Use
Prevent recurrence	Eliminates root causes rather than symptoms, blocking recurrence of the same problem	Make Fishbone analysis mandatory in post-incident post-mortems
Team collaboration	Shares causes from diverse perspectives across the team, eliminating blind spots	Use Fishbone workshops in quality circles and sprint retrospectives
Linked with Six Sigma	Used as a core tool in the Analyze phase of DMAIC	Narrow down top causes with a Pareto chart, then perform deep Fishbone analysis
Software quality improvement	Structural root-cause analysis of IT system failures and software defects	Applied to identify improvement initiatives in code review, testing, and deployment processes

Cost of Quality (CoQ)Pareto Principle