Precedence Diagramming Method (PDM) and Arrow Diagramming Method (ADM)

Lecture Notes: Precedence Diagramming Method (PDM) and Arrow Diagramming Method (ADM)

Introduction

Once project activities are defined and dependencies identified, project managers use diagramming methods to visualise the sequence and logic of tasks.
Two primary techniques are:

• Precedence Diagramming Method (PDM) — also called Activity-on-Node (AON).
• Arrow Diagramming Method (ADM) — also called Activity-on-Arrow (AOA).

Both methods serve the same purpose: to map how activities connect and depend on each other, but they differ in how they represent activities and relationships.

Purpose of Diagramming Methods

These methods help to:

• Visualise activity sequencing and dependencies.
• Identify the critical path (the longest sequence of dependent tasks).
• Support schedule analysis and what-if scenarios.
• Communicate project flow to stakeholders clearly.

 Precedence Diagramming Method (PDM)

Definition

PDM represents activities as nodes (boxes) and dependencies as arrows connecting those nodes.
It is the most widely used method today and is the standard for modern project management software like Microsoft Project and Primavera.

Alternate Name: Activity-on-Node (AON)

3.1 Features of PDM

• Activities are nodes or rectangles.
• Arrows indicate logical relationships (dependencies).
• Supports four types of relationships:
  1. Finish-to-Start (FS) – successor starts after predecessor finishes.
  2. Start-to-Start (SS) – successor starts after predecessor starts.
  3. Finish-to-Finish (FF) – successor finishes after predecessor finishes.
  4. Start-to-Finish (SF) – successor finishes after predecessor starts (rare).

3.2 Example (Finish-to-Start Relationship)

[Design] → [Develop] → [Test] → [Deploy]

Each box represents an activity, and arrows show dependencies.
Development begins only after Design is complete, and so on.

3.3 Advantages of PDM

• Simple and visually clear.
• Allows overlapping (parallel) activities using SS and FF relationships.
• Easy to modify when project scope or sequence changes.
• Supported by most project management tools.

3.4 Disadvantages of PDM

• May become complex for very large projects.
• Does not directly represent event milestones (start/finish points) like ADM does.

Arrow Diagramming Method (ADM)

Definition

ADM represents activities as arrows, while nodes (circles) represent start and finish events of those activities.
It is the original method used in the Critical Path Method (CPM) and PERT (Program Evaluation and Review Technique).

Alternate Name: Activity-on-Arrow (AOA)

4.1 Features of ADM

• Arrows represent the activities.
• Nodes (circles) represent events — the start or completion of one or more activities.
• Only one type of logical relationship is supported:
  • Finish-to-Start (FS)
    (The next activity starts only when the previous one finishes.)
• Dummy activities (dotted arrows) are sometimes used to show logic or dependencies that don’t consume time or resources.

4.2 Example

(1) ---A---> (2) ---B---> (3) ---C---> (4)

• Circles (1), (2), (3), (4) are events.
• Arrows (A, B, C) are activities.
• To show that Activity C depends on A but not B, a dummy activity (dotted arrow) might be introduced.

4.3 Advantages of ADM

• Clearly shows events and milestones.
• Provides a strong foundation for Critical Path and PERT analysis.
• Useful for milestone-based tracking.

4.4 Disadvantages of ADM

• Can only show Finish-to-Start relationships (no SS, FF, or SF).
• Requires dummy activities, which can clutter the diagram.
• More difficult to modify than PDM.
• Rarely supported by modern scheduling software.

Comparison Between PDM and ADM

 When to Use Each Method

• Use PDM when working with modern tools and projects that involve overlapping, concurrent, or flexible tasks.
• Use ADM for legacy systems, academic demonstrations of CPM/PERT, or when milestone-based tracking is central.

Example Comparison

PDM Representation

[Design] → [Develop] → [Test] → [Deploy]

ADM Representation

(1) ---Design---> (2) ---Develop---> (3) ---Test---> (4) ---Deploy---> (5)

 Summary

• PDM and ADM are tools for visualizing project activity sequences.
• PDM dominates modern practice due to its flexibility and clarity.
• ADM laid the foundation for Critical Path and PERT analysis but is now mostly of historical or educational value.

In short:

PDM is practical. ADM is classic. Both tell the same story — just in different languages.