Network Diagram Using Precedence Diagramming Method OR Activity On Node

Network diagram is primarily used to understand how the activities in a project are sequenced, understand the dependencies between activities and finally to understand the critical path in the project.

On the very simple terms, network diagram shows how the activities related to each other.

Meaning that in your project once you identify the activities and the order in which the activities needs to perform, you can represent this information in the form of network diagram.

Network Diagram Notations

There are primarily two type of notations to draw a network diagram. They are

1. Activity on Arrow (AOA) or Activity Diagramming Method (ADM)
2. Activity on Node (AON) or Precedence Diagramming Method (PDM)

Using The Activity on Arrow (AOA) or Activity Diagramming Method (ADM)

The activity on arrow (AOA) is less popular compared to the activity on node (AON) or precedence diagramming method (PDM).

Also another name for Activity on Arrow (AOA) diagram is activity diagramming method (ADM).

In this notation the activities and the duration of the activity is shown as an arrow (above the arrow). The nodes are shown to represent the dependencies between the activities.

OR

In the above diagram circles represent the nodes.

Activity A is shown on the arrow and 4 is the duration of the activity A.

The activity entering a node is a predecessor to the activity leaving the node. That way nodes are representing the dependencies in activity on arrow diagram notation.

In the above diagram activity A is a predecessor to activity B. Also if you closely observe, the duration of activity B is longer than the duration of activity A. Hence the arrow representing the activity B is a bit longer, when compare to activity A.

Sometimes the activity on arrow (AOA) diagrams use of inserting dummy. These dummy activities will have 0 duration activities.

So the arrow, in the activity on arrow diagram represents two attributes, one is sequence the other is duration. Representing sequence with a single arrow sometimes may not be feasible especially when you have complex structure of predecessors and successors. In those cases to represent the dependency and the sequence, the dummy activities will be used in the activity on arrow (AOA) diagrams.

Hold on. I did not understood the concept of dummy activities clearly. What is it about?

No worries. Nothing to be panic. We will see this concepts with an example in a while to understand the dummy activities in AOA diagrams in more detail.

Using The Activity on Node (AON) or Precedence Diagramming Method (PDM)

Now we will look at the popular method of creating the network diagram. Also called as Activity on Node diagram or Precedence diagramming method.

In activity on node (AON) or precedence diagramming method (PDM),

• Rectangles represent the nodes .
• And the project activity is shown on every node. For example A, B and C are the activities.
• The arrows represents the relation between the activities.
• Units of duration of the activity wrote above the node. For example 7 is the duration of the activity C.

Now we have seen both the diagramming methods to create the network diagram. Let us traverse through an example to understand how these two diagrams methods are different practically in creating the network diagram. Also let us see why dummy activities are essential in Activity on arrow (AOA) diagram.

Activity on Node and Activity on Arrow diagram Example

Let us say an IT project require the following activities to perform in the project.

Using the above table which represent the project activities, duration and the dependencies let us now create the network diagram.

Creating network diagram using Activity on arrow (AOA) or Activity diagramming method (ADM)

If you see the tricky part here is that C has the predecessor as A. And activity D has the predecessor activities A and B. This kind of representation is difficult to show in ADM. Let us see how it looks now.

If you carefully observe this is a wrong network diagram. The reason for this is, activity C has only one predecessor which is A. But in the above network diagram, activity C has two predecessors A and B.

Since the activity D has two predecessors, and there is no other way for activity B to represent that it is the predecessor only for activity D.

This is the issue with activity on arrow (AOA) or ADM network diagram. To resolve this issue, a dummy activity needs to insert as follows.

If you see in this activity on arrow (AOA) diagram, a dummy activity is there to show the dependency between activity A and activity D.

As mentioned earlier dummy activities have always 0 duration as the name says.

And if you see activity on arrow or activity diagramming method is a bit cumbersome when compare to the activity on node (AON) or precedence diagramming method (PDM) of creating the network diagram.

Creating network diagram using Activity On Node (AON) or Precedence Diagramming Method (PDM)

As mentioned earlier activity on node or precedence diagramming method (PDM) has become very popular because of its simplicity or less complex type of notations. For this reason lot of tool vendors use this as primary focus to create the network diagram.

Let us see now how to create network diagram using precedence diagram method or activity on node (AON) method.

In the above activity on node or precedence diagramming method, you can see the simplicity of the network diagram using PDM when compare to activity on arrow diagrams.

Conclusion

Though precedence diagramming method or activity on node diagram method is popular than the activity on arrow diagram, as a PMP Aspirant or project manager professional, you would need to know both the notations.

Hence in this blog post, I tried to touch the activity on arrow diagram also.

To conclude with network diagram is to represent the

• Order of the activities in the project
• Dependencies between the activities
• And to show the duration of the project activities

Finally to determine the critical path in the project and to analyze the activities on the critical path.