PDM vs AOA: Understanding Network Diagram Methods

This article explores the differences between two common network diagram methods used in project scheduling and time planning: PDM (Precedence Diagram Method) and AOA (Activity On Arrow). Let’s dive in!

There are two primary types of network diagrams:

• Activity on Arrow diagram (AOA) or Arrow Diagram (AD)
• Activity on Node diagram or Precedence diagram method (PDM)

Figure 1: Difference between PDM and AOA

As shown above, CPM/CPA (Critical Path Method/Critical Path Analysis) and PERT (Program Evaluation and Review Technique) fall under the Arrow network Diagram category. PDM, on the other hand, belongs to the Activity Network diagram or AON.

PDM - Precedence Diagram Method (AON)

Figure 2: PDM or AON

In PDM, as illustrated above, activities are represented by nodes or boxes. Arrows indicate the dependencies between these activities.

PDM recognizes four types of dependencies between activities:

• Finish to Start (FS): An activity must finish before its successor activity can start.
• Start to Start (SS): An activity must start before its successor activity can start.
• Finish to Finish (FF): An activity must finish before its successor activity can finish.
• Start to Finish (SF): An activity must start before its successor activity can finish. This type is rarely used.

AOA - Activity On Arrow (ADM)

Figure 3: AOA or ADM

In the AOA network diagram, activities are represented by arrows. Nodes, in this case, represent the dependencies between activities. Any activity leading into a node is a predecessor to any activity leaving the node.

AOA primarily uses the finish-to-start relationship between activities. Dummy activities might be introduced to clarify dependencies, even if they don’t require actual work or time.

As seen in the figure, there are two ways to denote an activity on an arrow in AOA: either the activity name is directly on the arrow, or the activity is named using the two nodes it connects (e.g., A-B). This method requires two-letter identifiers to name a single activity (e.g. A-B).

Key Differences Summarized