Tree Diagrams

Tree Diagrams in Six Sigma Black Belt: Define Phase

Understanding Tree Diagrams

Tree diagrams are powerful visual tools used in the Define phase of Six Sigma projects to break down complex problems into manageable components. They help teams systematically decompose broad business objectives into specific, actionable items.

What Are Tree Diagrams?

A tree diagram is a hierarchical visual representation that starts with a main objective or problem at the top (the trunk) and branches downward into increasingly detailed sub-objectives or solutions. Each level represents a further breakdown of the level above it, similar to an organizational chart or a family tree.

In Six Sigma, tree diagrams are also known as:

• Objectives Trees - when used to decompose goals
• Means-Ends Diagrams - when showing how means lead to ends
• Requirements Trees - when breaking down project requirements

Why Are Tree Diagrams Important?

Strategic Alignment: Tree diagrams ensure that all project activities align with overall business objectives by showing the connection between high-level goals and specific tasks.

Clear Communication: They provide a visual communication tool that helps all stakeholders understand how different components of a project relate to each other and to the ultimate goal.

Comprehensive Planning: By systematically breaking down objectives, teams ensure nothing is overlooked and all necessary elements are addressed.

Scope Definition: Tree diagrams help define project scope by clearly delineating what is and isn't included in the project.

Problem Decomposition: Complex problems become more manageable when broken into smaller, more understandable pieces.

Prioritization: The hierarchical structure helps teams prioritize initiatives based on their contribution to primary objectives.

How Tree Diagrams Work

Step 1: Identify the Primary Objective
Start by clearly defining the main goal or problem statement. This becomes the root or trunk of your tree. For example: Reduce Customer Wait Time

Step 2: Determine Major Categories
Identify the major factors or components that contribute to achieving the primary objective. These become the primary branches. Ask: What are the main ways we can reduce customer wait time?
Possible answers might include: Staffing, Process Efficiency, Technology, Training

Step 3: Break Down Further
For each major category, identify sub-components or specific actions. These become secondary and tertiary branches. Continue breaking down until you reach actionable items.

Step 4: Verify Completeness and Mutual Exclusivity
Ensure that:
  • All branches together comprehensively cover the parent objective
  • Branches don't overlap (mutual exclusivity)
  • The breakdown is logical and follows a consistent structure

Step 5: Connect to Actions
The final branches should be specific enough to translate into concrete actions, metrics, or deliverables.

Structure and Components

Root/Trunk: The primary objective or goal
Primary Branches: Major categories or critical success factors
Secondary Branches: Sub-categories or supporting elements
Tertiary Branches (Leaves): Specific actions, metrics, or implementation items

Example Tree Diagram

Primary Objective: Improve Customer Satisfaction

Primary Branches:
  1. Product Quality
     - Reduce defects
     - Enhance features
  2. Customer Service
     - Response time
     - Problem resolution
  3. Delivery
     - On-time delivery
     - Shipping accuracy

Applications in Define Phase

Tree diagrams are particularly valuable in the Define phase for:

Project Charter Development: Help articulate project goals and deliverables
Scope Definition: Clearly define what is in and out of scope
Stakeholder Communication: Explain how individual tasks contribute to overall objectives
Requirements Gathering: Systematically identify all project requirements
Metrics Development: Identify all areas that need measurement

Answering Exam Questions on Tree Diagrams

Common Exam Question Types

Type 1: Identification Questions
Question: Which of the following best represents the primary trunk of a tree diagram used to reduce manufacturing defects?
Approach: The trunk should be the ultimate objective, not a specific tactic. Look for the broadest, most inclusive statement.

Type 2: Application Questions
Question: A team wants to improve on-time delivery. Which would be an appropriate secondary branch?
Approach: Secondary branches should be major contributing factors, not final actions. Think about what categories of activities would impact on-time delivery.

Type 3: Structure Questions
Question: Why is it important that tree diagram branches are mutually exclusive?
Approach: This ensures no duplication of effort and provides clear delineation of responsibilities.

Type 4: Sequencing Questions
Question: In what order should you develop a tree diagram?
Approach: Always start with the primary objective, then work downward. Never start with tactics and work backward.

Exam Tips: Answering Questions on Tree Diagrams

Tip 1: Remember the Direction
Tree diagrams in Six Sigma flow from top to bottom, starting with the strategic objective at the top and becoming increasingly tactical as you move down. This is different from root cause analysis, which often flows upward. Always start with the what (objective) and work toward the how (implementation).

Tip 2: Focus on Hierarchy
When answering questions, ensure you understand the hierarchical relationship. Each lower level should be a means to achieve the level above. If a lower-level item doesn't contribute to its parent branch, it's incorrectly placed.

Tip 3: Apply the 80/20 Rule
Tree diagrams typically show the 20% of factors that drive 80% of results. When evaluating options, prefer comprehensive but focused breakdowns that capture vital elements without excessive detail.

Tip 4: Use Logical Grouping
All branches at the same level should be of similar type and granularity. If you have branches like Customer Service, Product Quality, and John's Responsibilities, something is wrong. Maintain consistent categorization.

Tip 5: Verify Completeness
For questions asking whether a tree diagram is complete, check that:
  • Every primary branch is necessary to achieve the primary objective
  • Together, the branches comprehensively cover the objective
  • No significant gaps exist

Tip 6: Distinguish from Other Tools
Don't confuse tree diagrams with:
  • Fishbone diagrams: Used for root cause analysis (causes and effects)
  • Process maps: Show sequential flow of activities
  • Pareto charts: Show relative importance through frequency
If the question mentions a tree diagram specifically, resist the urge to answer using other analysis tools.

Tip 7: Look for Actionability
The most detailed branches should be actionable. If you see a tree diagram where the leaves are still vague statements rather than specific, measurable items, that's incomplete decomposition.

Tip 8: Consider the Purpose
Tree diagrams have different purposes:
  • Objectives Trees: Decompose goals into sub-goals
  • Means-Ends Trees: Show causality and relationships
  • Requirements Trees: Break requirements into specifications
Understanding the purpose helps you correctly structure and interpret the diagram.

Tip 9: Practice with Context
When answering exam questions, consider the business context. Not all possible breakdowns are appropriate—choose the one that makes sense for the specific business problem.

Tip 10: Watch for Red Herrings
Exam questions often include plausible-sounding but incorrect branches. These might be:
  • At the wrong level of detail
  • Overlapping with other branches
  • Outside the scope of the primary objective
  • Solutions rather than objective decompositions
Read carefully and think critically.

Sample Exam Questions and Strategies

Question 1: A Black Belt is creating a tree diagram to improve delivery performance. The primary objective is at the top. Which of the following would be most appropriate as a secondary branch (first level branch)?
  A) Install new warehouse management system
  B) Reduce freight costs
  C) Improve demand forecasting
  D) Train warehouse staff

Strategy: Options A and D are too specific (they're tactics/actions, not categories). Option B is about cost, not delivery performance. Option C is a category of activities that impacts delivery. Answer: C

Question 2: Which characteristic ensures a tree diagram avoids duplication of effort and provides clear accountability?
  A) Vertical alignment
  B) Mutual exclusivity
  C) Horizontal formatting
  D) Equal depth

Strategy: Mutual exclusivity means each branch is distinct with no overlap. This prevents duplication and clarifies responsibilities. Answer: B

Question 3: A tree diagram for a project has been developed. The primary objective is broken into three primary branches, which are further broken into secondary branches. A team member suggests adding a secondary branch under Primary Branch A that actually belongs under Primary Branch B. What principle is being violated?
  A) Parsimony
  B) Mutual exclusivity
  C) Completeness
  D) Measurability

Strategy: Placing an item in the wrong category violates the principle that branches should be mutually exclusive—each branch should cover distinct territory. Answer: B

Key Formulas and Concepts to Remember

Tree Diagram Levels:
  Level 1 (Root): Primary objective or goal
  Level 2 (Primary Branches): Major categories or critical success factors
  Level 3+ (Secondary/Tertiary Branches): Increasingly specific sub-items
  Final Level (Leaves): Actionable items or measurable metrics

Validation Criteria:
  ✓ Completeness: All major factors are included
  ✓ Mutual Exclusivity: No overlap between branches
  ✓ Hierarchy: Each lower level supports the level above
  ✓ Actionability: The most detailed levels are specific enough to act upon
  ✓ Relevance: Every branch is necessary to achieve the parent objective

Common Mistakes to Avoid

Mistake 1: Confusing Tree Diagrams with Root Cause Analysis
Tree diagrams decompose objectives; fishbone diagrams find root causes. Know the difference for exam questions.

Mistake 2: Starting with Tactics Instead of Objectives
Always start with the strategic objective and work downward. Never start with implementation details.

Mistake 3: Mixing Levels of Abstraction
All branches at the same level should be of similar scope and type. Don't mix strategic and tactical items at the same level.

Mistake 4: Overlapping Branches
Ensure branches are mutually exclusive. Items shouldn't fit into multiple branches.

Mistake 5: Incomplete Decomposition
Continue breaking down until you reach actionable items, but don't over-decompose into irrelevant detail.

Mistake 6: Failing to Validate
Always verify that together, all primary branches comprehensively cover the primary objective with no significant gaps.

Practice Exercise

Scenario: You're a Black Belt working on a project to reduce customer complaint handling time. Create a partial tree diagram and identify the primary objective, appropriate primary branches, and at least one secondary branch under each.

Answer Structure:
Primary Objective: Reduce customer complaint handling time
Primary Branches (suggested):
  1. Complaint Receipt and Routing
     - Streamline intake process
     - Improve categorization
  2. Resource Availability
     - Increase staff capacity
     - Improve staff utilization
  3. Problem Resolution
     - Enhance troubleshooting capabilities
     - Expand authority to resolve
  4. Communication and Follow-up
     - Establish communication protocols
     - Implement tracking system

This exercise helps you practice hierarchical thinking and logical decomposition—essential skills for exam success.