Activity Network Diagrams: Complete Guide for Six Sigma Black Belt

Activity Network Diagrams: Complete Guide for Six Sigma Black Belt

What is an Activity Network Diagram?

An Activity Network Diagram (AND), also known as a Project Network Diagram or PERT Chart, is a visual representation of all the activities, tasks, and milestones involved in a project. It displays the sequential relationships between tasks and shows which activities must be completed before others can begin. The diagram illustrates dependencies, critical paths, and timelines for project completion.

Why is it Important?

Activity Network Diagrams are crucial in project management and Six Sigma because they:

• Identify Critical Path: Shows the longest sequence of dependent activities that determines the minimum project duration
• Improve Planning: Helps project managers schedule activities and allocate resources efficiently
• Reduce Project Duration: Allows identification of parallel activities that can run simultaneously
• Risk Management: Helps identify potential delays and bottlenecks early
• Communication: Provides a clear visual representation for stakeholders and team members
• Track Progress: Enables comparison of actual progress against planned timelines
• Resource Allocation: Helps balance workload and identify resource constraints

How it Works: Key Concepts

Components of an Activity Network Diagram

1. Activities/Tasks: The work that must be performed. Represented as:

• Boxes (in Activity-on-Node method)
• Arrows (in Activity-on-Arrow method)

2. Nodes/Vertices: Points in time representing the start or completion of activities

3. Arrows/Edges: Show dependencies and the flow of the project (Activity-on-Arrow method)

4. Dependencies: Relationships between activities that determine the sequence

Types of Dependencies

Finish-to-Start (FS): Activity B cannot start until Activity A is finished (most common)

Start-to-Start (SS): Activity B cannot start until Activity A starts

Finish-to-Finish (FF): Activity B cannot finish until Activity A finishes

Start-to-Finish (SF): Activity B cannot finish until Activity A starts (rarely used)

Two Main Methods

Activity-on-Node (AON): Activities are represented in boxes/nodes, arrows show dependencies

Activity-on-Arrow (AOA): Activities are represented on arrows, nodes represent events or milestones

Time Calculations: Forward and Backward Pass

Forward Pass: Calculates the Earliest Start (ES) and Earliest Finish (EF) times

• ES of first activity = 0
• EF = ES + Duration
• ES of next activity = EF of previous activity

Backward Pass: Calculates the Latest Start (LS) and Latest Finish (LF) times

• LF of last activity = EF of last activity
• LS = LF - Duration
• LF of previous activity = LS of next activity

Slack/Float: The amount of time an activity can be delayed without affecting the project completion date

• Slack = LS - ES or LF - EF
• Activities with zero slack are on the critical path

Critical Path

The critical path is the longest sequence of dependent activities that determines the shortest possible project duration. Activities on the critical path have zero slack and cannot be delayed without delaying the entire project.

How to Create an Activity Network Diagram

Step-by-Step Process

Step 1: Identify All Activities
List all tasks required to complete the project

Step 2: Determine Durations
Estimate the time required for each activity

Step 3: Determine Dependencies
Identify which activities must be completed before others can start

Step 4: Create the Diagram
Draw nodes and arrows showing the sequence and relationships

Step 5: Calculate Times
Perform forward and backward passes to calculate ES, EF, LS, LF, and slack

Step 6: Identify Critical Path
Highlight the sequence of activities with zero slack

Step 7: Analyze and Optimize
Look for opportunities to compress the timeline or manage resources

Exam Tips: Answering Questions on Activity Network Diagrams

Common Question Types

1. Identifying the Critical Path

• Look for the longest sequence of dependent activities
• Find activities where Slack = 0 or LS = ES
• Remember: Critical path determines minimum project duration
• Tip: Draw the network clearly and calculate all times systematically

2. Calculating Project Duration

• The project duration equals the EF of the last activity on the critical path
• Don't add all activity durations; follow dependencies
• Tip: Only parallel paths can run simultaneously

3. Computing Slack/Float

• Use formula: Slack = LS - ES (or LF - EF)
• Zero slack indicates critical activities
• Positive slack shows flexibility in scheduling
• Tip: Double-check your forward and backward pass calculations before computing slack

4. Interpreting Dependencies

• Understand the relationship types (FS, SS, FF, SF)
• Most exam questions use Finish-to-Start relationships
• Tip: Ask "Can this activity start before that one?" to determine dependencies

5. Resource Leveling Questions

• Identify activities that can be delayed due to slack
• Understand how to reallocate resources to non-critical activities
• Tip: Cannot delay critical path activities without extending project duration

Examination Strategies

Strategy 1: Systematic Approach
Always follow a methodical process: list activities, determine durations, identify dependencies, create diagram, calculate times

Strategy 2: Double-Check Calculations
Verify forward pass before performing backward pass. Ensure EF of one activity matches ES of dependent activity

Strategy 3: Highlight the Critical Path
Use color or bold lines to clearly mark activities with zero slack. This helps visualize the answer and prevents errors

Strategy 4: Understand the Purpose
Remember why AND is used in Six Sigma: to optimize project timelines, reduce waste, and improve efficiency. Questions often test this application

Strategy 5: Practice with Real Scenarios
Six Sigma Black Belt exams often present realistic project scenarios. Understand how AND helps identify improvement opportunities

Strategy 6: Manage Time During Exam
Complex AND calculations can be time-consuming. Allocate time wisely and don't get stuck on one question

Strategy 7: Know Key Formulas
Memorize these formulas for exam day:

• EF = ES + Duration
• LS = LF - Duration
• Slack = LS - ES
• Project Duration = Maximum EF of all activities

Common Mistakes to Avoid

• Adding all durations: Remember parallel activities don't add; only dependent sequences do
• Ignoring dependencies: Carefully read which activities must be completed first
• Wrong slack calculation: Ensure you subtract in the correct direction (LS - ES, not ES - LS)
• Assuming all activities are critical: Only those with zero slack are on the critical path
• Not updating times after changes: If activities are compressed or delayed, recalculate all dependent activities
• Confusing activity duration with project duration: Project duration is the sum of critical path, not all activities

Sample Exam Question Format

Example Question:
"In a process improvement project, you have the following activities and dependencies: Activity A (5 days), Activity B (3 days, depends on A), Activity C (4 days, depends on A), Activity D (2 days, depends on B and C). What is the project duration and which activities are on the critical path?"

Approach:

1. Create the network: A → B and C (parallel), then B and C merge → D
2. Calculate: A (0-5), B (5-8), C (5-9), D (9-11)
3. Project duration = 11 days
4. Critical path = A → C → D (5+4+2=11 days)
5. Activity B has 1 day slack (9-8=1)

Advanced Tips for Black Belt Level

• Three-Point Estimation: Use Optimistic, Most Likely, and Pessimistic durations to calculate Expected Duration = (O + 4M + P) / 6
• Resource Constraints: Understand how resource limitations affect the network and may create additional dependencies
• Risk Analysis: Consider how uncertainties in activity durations affect project completion and critical path stability
• Process Improvement Context: Connect AND to DMAIC phases; AND is most relevant in Control phase for project management
• Simulation Tools: Know that Monte Carlo simulation can be applied to AND when dealing with uncertain durations

Key Takeaways

Activity Network Diagrams are essential tools in Six Sigma for project planning, scheduling, and control. For the Black Belt exam, focus on:

• Understanding dependencies and how they affect project flow
• Calculating ES, EF, LS, LF, and slack correctly
• Identifying the critical path and its significance
• Recognizing parallel activities and opportunities for compression
• Applying AND to real process improvement scenarios
• Managing resources and risk based on network analysis

With consistent practice and a systematic approach to problem-solving, you will confidently answer AND questions on your Black Belt certification exam.