Activity Network Diagrams

5 minutes 5 Questions

Activity Network Diagrams (AND), also known as Project Network Diagrams or PERT charts, are critical tools used during the Define Phase of Lean Six Sigma projects to visualize project scope, timeline, and dependencies. In the context of a Black Belt certification, understanding AND is essential for effective project planning and management. An Activity Network Diagram displays the sequence of project activities, their logical relationships, and the critical path—the longest sequence of dependent activities that determines the minimum project duration. Each activity is represented as a node or arrow, connected to show how tasks must flow and which activities can occur simultaneously. Key components include nodes representing activities or events, arrows showing task sequences and dependencies, and values indicating duration. The diagram identifies four types of relationships: Finish-to-Start (most common), Start-to-Start, Finish-to-Finish, and Start-to-Finish. During the Define Phase, AND serves multiple purposes. First, it clarifies project scope by mapping all necessary activities and their relationships. Second, it enables accurate timeline estimation by identifying parallel and sequential tasks. Third, it highlights the critical path, allowing Black Belts to focus improvement efforts on activities that impact project completion. AND also facilitates resource allocation and risk identification. By visualizing task dependencies, project managers can allocate resources efficiently and identify potential bottlenecks. Additionally, understanding which activities have slack time helps determine where delays are tolerable versus where tight control is necessary. For Lean Six Sigma practitioners, AND integrates with other Define Phase tools like SIPOC diagrams and project charters. It provides a realistic baseline for project scheduling and helps stakeholders understand project complexity. Effective use of Activity Network Diagrams during Define Phase sets the foundation for successful project execution, ensuring all team members understand timelines, dependencies, and critical success factors, ultimately leading to more controlled and efficient improvement initiatives.

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.

Test mode:
Start practice test

CSSBB - Activity Network Diagrams Example Questions

Test your knowledge of Activity Network Diagrams

Question 1

What is the primary difference between Activity-on-Arrow (AOA) and Activity-on-Node (AON) representations in Activity Network Diagrams?

Correct Answer: AOA places activities on arrows between nodes, while AON places activities on the nodes themselves

In Activity Network Diagrams, the fundamental distinction is that Activity-on-Arrow (AOA) represents activities as arrows (lines) connecting event nodes, while Activity-on-Node (AON) represents activities directly on the nodes with arrows simply showing the logical dependencies between them.

The second option reverses the AOA definition incorrectly. The third option conflates network scheduling calculations (forward and backward pass) with the diagram format, which is unrelated to how activities are represented. The fourth option incorrectly describes both methods in terms of predecessor and successor relationships, which are features of scheduling logic, not diagram representation style.

Key concept tested: understanding the two conventions for representing project activities in network diagrams.

Question 2

In an Activity Network Diagram, what distinguishes a dummy activity from a real activity?

Correct Answer: It consumes zero time and resources but shows logical dependency between events

A dummy activity in an Activity Network Diagram (specifically in Activity-on-Arrow notation) is a dashed arrow that consumes zero time and zero resources, existing solely to represent a logical dependency or precedence relationship between events without duplicating activity identifiers.

The buffer option confuses dummy activities with project buffer concepts from Critical Chain methodology. The constraint option incorrectly attributes minimal time and resource consumption to dummy activities, which by definition consume none. The milestone option confuses dummy activities with milestone markers; milestones are zero-duration events, but they are not the same as dummy activities, which specifically address dependency logic.

Key concept tested: understanding dummy activities in Activity-on-Arrow network diagramming.

Question 3

What is the other commonly used name for an Activity Network Diagram in project management?

Correct Answer: Arrow Diagram Method (ADM)

The Activity Network Diagram is also commonly known as the Arrow Diagram Method (ADM), where activities are represented on the arrows connecting nodes, making it one of the seven management and planning tools used in Lean Six Sigma and project management.

The Precedence Diagram Method is a distinct technique where activities are represented on the nodes rather than arrows. PERT is a broader scheduling analysis technique that uses probabilistic time estimates, not simply another name for the Activity Network Diagram. Activity-on-Node is the format used in PDM, which is a different representation approach from the arrow-based network diagram.

The key concept tested is knowledge of project management diagramming terminology and the seven management and planning tools.

More Activity Network Diagrams questions
27 questions (total)
Start 27 question test

Other Define Phase concepts

Affinity DiagramsBusiness Case JustificationCritical-to-Quality (CTQ) RequirementsCTQ TreeCustomer Data Collection MethodsCustomer Identification and SegmentationGantt ChartsInterrelationship DigraphKano ModelMatrix DiagramsPrioritization MatricesProblem Statement DevelopmentProcess Decision Program Chart (PDPC)Project Charter ReviewProject Performance MeasurementsProject Scope DefinitionQuality Function Deployment (QFD)RACI ModelSIPOC DiagramSMART Goals and ObjectivesSurveys, Focus Groups, and InterviewsToll-Gate ReviewsTree DiagramsWork Breakdown Structure (WBS)
All Define Phase questions · CSSBB practice test