What is a Project Schedule Network Diagram?

Project Management Professional (PMP)® certification requires knowledge of the project schedule network diagram in terms of types, benefits, and how to create one. PMP credential holders should also know how to use the project schedule network diagram to identify the critical path and float for the project to enhance all schedule management.

On this page:

• What is a Project Schedule Network Diagram PMP?
• Project Network Diagram Terms
• Types of Project Network Diagrams
• How to create a Project Network Diagram
• Benefits of using a Project Network Diagram
• Summary

What is a Project Schedule Network Diagram PMP?

Several terms refer to network diagrams within project management. They all refer to the visual representation of the project schedule. For example:

• Project schedule network diagram PMP
• Network diagram
• Project network diagram
• Project schedule network diagram
• Schedule network
• Schedule network diagram

Within Project Management and the PMP certification exam, a project schedule network diagram is a graphical representation of the logical relationships among the project schedule activities derived from the project schedule. It will:

• provide a visual representation of the interconnectivity across project activities,
• use activities, activity duration, and interdependencies to show the overall project schedule,
• use rectangles, circles, solid lines, dotted lines, and arrows to represent schedule components and their relationships, and
• serve as a tool to communicate schedule progress and issues.

For example, Project Managers use a project network diagram to identify the critical path and float for the project to understand the overall project schedule better. Additionally, the project manager uses the project network diagram to communicate progress against the planned project schedule. 

Project Network Diagram Terms

A project schedule network diagram requires understanding key terms and applying them to the network diagram. For example,

• Activity Dependency: relationships among activities that shape the sequence in which the activities can be started and must be completed within the overall schedule.
• Arrows: graphical representation of activity connection, sequence, and interdependency
• Critical Path: the longest sequence of activities that must be completed for the project to be finished on time.
• Float: the amount of time an activity can be delayed without affecting a successor activity (free float) or the amount of time an activity can be delayed without affecting the project end date (total float)
• Lag: the amount of time needed between two dependent activities
• Lead: the amount of time an activity can be accelerated in relation to a preceding activity
• Predecessor: an activity that must happen before another can begin
• Successor: an activity that must happen after a previous activity concludes

This list is not exhaustive but addresses the core terms and concepts within project network diagrams.

Types of Project Network Diagrams

Project schedule network diagrams are “Activity on Arrow” or “Activity on Node.” The main difference between an “activity on arrow” diagram and an “activity on node” diagram is the level of detail. An activity on arrow diagram shows the logical relationships between activities, while an activity on node diagram also includes the duration of each activity.

Therefore, as the activity on node diagram is more detailed, it is typically used during a project’s planning phase but can also be used during the execution phase.

Schedule Network Diagram Type 1: Activity on Arrow / Arrow Diagram / Arrow Diagram Method (ADM)

Activity on arrow uses arrows to represent project activities: the end of the arrow is the activity start and connects to a node (box), and the head (the pointed end) is the activity end that connects to a node (box). The arrow length represents the activity duration. ADM can only show “finish-to-start” (FS) relations among activities.

The limitations of the activity on arrow diagram led to its limited use in project management. However, as an arrow diagram may appear in limited use in some businesses, they are a potential topic in PMP certification exam questions.

Schedule Network Diagram Type 2: Activity on Node / Precedence Diagram Method (PDM)

The preferred and most common network diagram tool is the Activity on Node or precedence diagram. Project Management Institute (PMI) defines the precedence diagram as: “A technique used for constructing a schedule model in which activities are represented by nodes and are graphically linked by one or more logical relationships to show the sequence in which the activities are to be performed.”

An Activity on Node Diagram, also known as the Precedence Diagram Method (PDM), lacks the limitation of the arrow diagram. For example, activity on node can:

• Capture lead time among activities
• Show lag time among activities
• Incorporate all four activity dependency types. The four dependency types are:

1. Finish-To-Finish (FF): one activity cannot finish until the predecessor activity is finished; for example, activity A must be finished before activity B can be finished.
2. Finish-to-Start (FS): one activity cannot start until the other ends; for example, activity A cannot start until activity B is finished.
3. Start-to-Finish (SF): one activity cannot finish until the predecessor activity has started; for example, activity A must start before we can finish activity B.
4. Start-to-Start (SS): one activity cannot start until a predecessor activity has started; for example, activity A cannot start unless B has started.

The ability to accurately convey all activity dependencies makes the precedence diagram a more practical and widely used schedule network tool.

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How to create a Project Network Diagram

To create a Project Schedule Network Diagram, begin by listing all the activities that need to be completed for the project. Then, draw arrows between the activities to show the logical relationships. Finally, add the duration of each activity.

1. Define activities (be thorough in encompassing the entire project)
2. Estimate the activity durations (determine how long each activity will take; use team and subject matter expertise to make estimations)
3. Sequence the activities (map out activities in the order they should / can happen)
4. Create the project network diagram (most project scheduling software can create a diagram using your submitted data)

When do you create a Project Network Diagram?

A project schedule network diagram is typically created during the planning phase of a project. However, it can also be created during the execution phase if changes to the project occur that require an updated diagram.

Benefits of using a Project Network Diagram

As with any project management planning work, the accuracy of the information used will impact the effectiveness of the resulting resource or document. With a well-constructed project schedule network diagram, also called a project network diagram, the Project Manager is better positioned to complete a variety of tasks. For example:

• able to identify all activities needed for project success
• quickly recognize the sequence of tasks with the longest duration and establish a critical path
• provide the project team and stakeholders with a visual representation of the project schedule
• track activity dependencies and potential risks, and
• share the activity schedule status with the team and stakeholders.

There are challenges and risks with using a project schedule network diagram. For example: 

• the time used to create an accurate project schedule network could be used elsewhere in project management tasks, and 
• a misinterpretation of the network diagram can lead to costly mistakes.

Project managers use the network diagram to enhance their schedule management work. However, it should be only one of many schedule information sources and tools utilized throughout the project.

Summary

A Schedule Network Diagram is a graphical representation of the logical relationships among the project schedule activities. Use this diagram to identify the critical path and float within the project schedule. The most used project network diagram type is the activity on node, also called a precedence diagram. It captures all project activity dependency types, activity leads and lags, and free and total float, in addition to the critical path. Knowledge of the benefits, uses, and components of the project schedule network diagram should be part of your study plan when preparing for the PMP certification exam.

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