Cause and Effect Diagrams and
Root Cause Analysis

What is covered in this Article

Cause and effect diagrams and root cause analysis provide structured problem-solving methodologies essential for quality engineering and process improvement. Within the Analyze Phase of the Six Sigma DMAIC framework, identifying the fundamental root causes of process defects rather than treating surface symptoms is critical. By deploying cause and effect diagrams—widely known as Ishikawa or fishbone diagrams—quality teams systematically structure brainstorming sessions, categorize potential variables, and link them directly to observed process effects.

Historical Origins and the Ishikawa Framework

Developed by Dr. Kaoru Ishikawa of the University of Tokyo in the 1940s and popularized through JUSE training initiatives, the Cause-and-Effect Diagram was created to visualize the complex interactions of variables in manufacturing processes. Ishikawa believed that empowering cross-functional teams to map out potential causes visually would democratize quality control and unlock shop-floor problem-solving capabilities.

Framework Domain	Standard Category Branches
Manufacturing (6Ms)	Manpower, Machine, Material, Method, Measurement, Mother Nature
Service & Administrative (4Ps)	Policies, Procedures, People, Plant / Platform

Mathematical and Logical Structuring of Root Cause Analysis

While fishbone diagrams organize qualitative brainstorming, rigorous root cause analysis relies on quantitative verification and structured probabilistic ranking. To evaluate the relative contribution of validated causes ( $C_i$ ) to total process variation ( $V_t$ ), quality engineers utilize structured variance partitioning:

$\displaystyle V_t = \sum_{i=1}^{m} \sigma^2(C_i) + \sigma^2_{error}$

Where:

$\sigma^2(C_i)$ = Variance contribution of the $i$ -th classified root-cause branch
$\sigma^2_{error}$ = Unexplained common-cause process variation

Worked Numerical Example: Multi-Branch Variance Contribution

A quality team analyzes dimensional variability yielding a total variance ( $V_t$ ) of $10.0 \text{ mm}^2$ across four major fishbone branches: Machine (4.5), Method (3.0), Material (1.5), and Measurement (1.0).

Fishbone Category Branch	Variance Contribution ( $\sigma^2$ )	Percentage Share
Machine (Primary Source)	4.5	45.0%
Method	3.0	30.0%
Material	1.5	15.0%
Measurement	1.0	10.0%

Elaborate Interpretation of Root Cause Analysis

Interpreting fishbone diagrams and root cause investigations requires disciplined analytical rigor:

Separating Symptoms from Causes: Teams must avoid listing superficial effects on bone branches. For instance, “machine breakdown” is a symptom; the underlying root cause is “lack of preventive lubrication maintenance.”
The Five Whys Integration: Pairing fishbone brainstorming with the “Five Whys” technique drills down through secondary and tertiary branches to uncover systemic failures.
Cross-Functional Validation: Every potential cause generated during diagramming must be tested with empirical data rather than accepted based on seniority or consensus.

Graphical Best Practices for Quality Engineering

Clear Effect Statement: Place the precise problem statement in a box on the far right as the “head” of the fishbone skeleton.
Balanced Branching: Ensure all major 6Ms or 4Ps branches are explored even if initial brainstorming focuses heavily on a single category like machines.
Hierarchical Detail: Attach secondary and tertiary contributing factors cleanly as smaller ribs branching off the main category bones.

Transitioning into Process Improvement

Completing root cause analysis within the Analyze Phase provides validated corrective targets. Teams transition smoothly into the Improve Phase, where solutions are tested and implemented to permanently eliminate identified assignable causes.

This content aligns with professional quality engineering body-of-knowledge standards.

Written by Ravi Prakash—Quality Expert (38+ yrs exp). Connect on LinkedIn or Contact Us.

Frequently Asked Questions (FAQ)

Q1: What is the primary purpose of a Cause and Effect Diagram in Six Sigma?

A Cause and Effect Diagram structures team brainstorming sessions to systematically identify, categorize, and explore all potential root causes contributing to a specific quality problem or defect effect.

Q2: What do the 6Ms represent in a manufacturing fishbone diagram?

The 6Ms stand for Manpower, Machine, Material, Method, Measurement, and Mother Nature (Environment), serving as the standard categorical branches for organizing potential manufacturing causes.

Q3: How does the Five Whys technique complement fishbone diagrams?

The Five Whys technique is used to drill down deeply along a specific fishbone branch, repeatedly asking “Why?” to move past surface symptoms down to foundational systemic root causes.

Q4: In which DMAIC phase are cause and effect diagrams primarily deployed?

Cause and effect diagrams are primarily deployed during the Analyze Phase to investigate and isolate the vital root causes identified by earlier measurement and prioritization tools.

Six Sigma Practice Exam Questions

During a brainstorming session for a surface finish defect, a team categorizes potential variables under Manpower, Machine, Material, Method, Measurement, and Environment. Which tool are they utilizing?
- A) Pareto Chart
- B) Cause and Effect Diagram
- C) Scatter Diagram
- D) Control Chart
Correct Answer: B) Cause and Effect Diagram
Explanation: The 6Ms framework is the classic structuring mechanism for an Ishikawa fishbone diagram.
A quality team calculates process variance contributions totaling , with the Machine branch contributing . What is the percentage variance share of the Machine branch?
- A) 25.0%
- B) 40.0%
- C) 50.0%
- D) 75.0%
Correct Answer: C) 50.0%
Explanation: $\displaystyle \text{Share} = \left( \dfrac{6.0}{12.0} \right) \times 100\% = 50.0\%$ .
In the context of root cause analysis, what is the fundamental difference between a symptom and a root cause?
- A) Symptoms are numerical while root causes are categorical.
- B) Symptoms are observable effects of a problem, whereas root causes are the underlying systemic failures producing the effect.
- C) Root causes can only be identified using control charts.
- D) Symptoms are eliminated in the Control phase while root causes are addressed in Define.
Correct Answer: B) Symptoms are observable effects of a problem, whereas root causes are the underlying systemic failures producing the effect.
Explanation: Effective root cause analysis moves past superficial symptoms to resolve foundational process failures.
Which of the 4Ps framework categories is typically applied in administrative and service sector fishbone diagrams instead of traditional manufacturing branches?
- A) Plant, Policies, Procedures, People
- B) Price, Product, Promotion, Place
- C) Profit, Productivity, Performance, Planning
- D) Parts, Packaging, Pressure, Personnel
Correct Answer: A) Plant, Policies, Procedures, People
Explanation: The 4Ps (Policies, Procedures, People, Plant/Platform) adapt fishbone analysis for service and transactional environments.
What is the recommended placement of the problem statement on a completed Cause and Effect Diagram?
- A) In the top left corner as an executive summary
- B) On the far right enclosed in a box as the head of the fishbone skeleton
- C) Along the bottom baseline beneath the major category branches
- D) In a separate supporting document rather than on the diagram
Correct Answer: B) On the far right enclosed in a box as the head of the fishbone skeleton
Explanation: The effect or problem statement forms the head of the fishbone, with major category bones feeding horizontally into the central spine.

Continue Following the Masterclass

If you would like to follow this masterclass series on quality engineering and process statistics, look at the Previous Post: 1026. Pareto Principle and Pareto Analysis for Quality Engineering Excellence and the Next Post: 1028. Process Flowcharts and Stratification Methods.

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