Network Diagrams: AON vs AOA for the PMP Exam Explained

Network Diagrams on the PMP Exam: AON Is King

Network diagrams are visual representations of project schedule logic, showing how activities are sequenced and related. The PMP exam recognizes two historical formats — Activity-on-Node (AON) and Activity-on-Arrow (AOA) — but you need to know that AON is the method PMI uses exclusively. Understanding why, and knowing the key differences, protects you from exam traps.

Activity-on-Node (AON)

In an AON diagram, activities are represented as boxes (nodes), and arrows represent the dependencies between them. This is the format used by the Precedence Diagramming Method, which is the only method recognized in the current PMBOK Guide for creating schedule network diagrams.

Strengths of AON

• Supports all four dependency types: FS, FF, SS, and SF.
• Does not require dummy activities (a significant advantage over AOA).
• Easier to read and construct for complex projects.
• Used by all modern scheduling software (Microsoft Project, Primavera, etc.).

How to Read an AON Diagram

Each node typically contains the activity name, duration, early start, early finish, late start, late finish, and float. Arrows between nodes show the direction and type of dependency. The critical path runs through nodes with zero float.

Activity-on-Arrow (AOA)

In an AOA diagram (also called Arrow Diagramming Method or ADM), activities are represented as arrows, and nodes represent the start or end points (events) of activities. This method is older and less flexible than AON.

Limitations of AOA

• Only supports finish-to-start relationships. It cannot represent SS, FF, or SF dependencies.
• Requires dummy activities (dashed arrows with zero duration) to correctly show certain dependency relationships. Dummy activities have no work or duration — they exist solely to maintain diagram logic.
• More difficult to construct and update for complex schedules.
• Not used in the current PMBOK Guide or by modern PM software.

Why AOA Still Appears on the PMP Exam

Although PMI no longer recommends AOA, the PMP exam may still reference it to test whether you understand the distinction. Questions might:

• Ask which diagramming method requires dummy activities (answer: AOA/ADM).
• Ask which method supports all four dependency types (answer: AON/PDM).
• Present a diagram and ask you to identify it as AON or AOA based on its structure.
• Ask which method is used in the Sequence Activities process (answer: PDM, which uses AON).

Dummy Activities Explained

Dummy activities are unique to AOA diagrams. They are needed when two activities share the same start and end nodes but have different dependencies. The dummy activity creates a separate logical path to preserve the correct dependency structure.

For example, if Activity A and Activity B both start at Node 1 and end at Node 2, but Activity C depends only on Activity A (not B), a dummy activity is inserted to separate the two paths. Without it, the diagram would incorrectly show C dependent on both A and B.

Constructing an AON Network Diagram for the PMP

If a PMP question asks you to build or analyze a network diagram, follow this process:

1. List all activities and their dependencies.
2. Draw a node for each activity.
3. Connect nodes with arrows showing the dependency direction (predecessor → successor).
4. Label each arrow with the dependency type (FS, SS, FF, SF) and any lead or lag.
5. Perform the forward pass to calculate ES and EF for each node.
6. Perform the backward pass to calculate LS and LF.
7. Calculate float for each activity. Zero-float activities form the critical path.

Exam Strategy

When you see a network diagram question on the PMP exam, first identify whether it is AON or AOA. If activities are in boxes, it is AON. If activities are on arrows, it is AOA. Then apply the appropriate rules for that format. In nearly all cases, the exam uses AON.

Practice building and analyzing network diagrams with our interactive PMP prep tools.