Q 491. Network Diagrams depict the sequence of activities and their relationships in a project. There are two different ways in which this can be done - 'Activity on Arrow' and 'Activity on Node'. Which one will you prefer to use and why (support your answer with illustrations preferably using the same data). 

 

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Activity on Arrow (AOA), as the name implies, these network diagrams denote each activity as an arrow. Here the nodes denote the start and end of an activity.  

Activity on Node (AON), this method is also referred as Precedence Diagram Method (PDM), and here the arrow represents the logical relationship amounts the activities and the nodes denote activities.

Decision:
Even though both methods accomplish same results, practitioners prefer AON over AOA, as it does not require the use of dummy activities.

Dummy activity (connecting link) is an imaginary activity, which does not require any time not any resource, still used to identify the dependence among operation. Whilst we can maintain the network Logic and avoid opacity (difficulty in interpretation).  

 

In AON, as activities are represented by nodes and its interdependency can be directly denoted by connected arrows, accordingly there is no need of dummy activity.  

Considerations: 

• AOA, can have several separate possible networks illustrating the same project, contra AON representations are unique  
• AON diagrams are comparatively easier to create and interpret  
• When it comes to amendments, design changes are easier in AON, then doing the same on AOA structure 
• AON focuses on tasks, whilst AOA focuses on events 
• Leaders every so often, get confused with AOA networks and prefer to see more of AON representations 

Lets consider the below example and design the respective AOA and AON structures. 

Choose Project, A, 1
Discovery, B, 3 
Get Go ahead, C, 2 
Data Collection, D, 3
Assemble team and Kickoff, E, 2
Finalize actionable, F, 3 
Leadership Summary, G, 1 

Although AON is advantageous, it becomes challenging under the below listed situations:
Path tracking by activity number is hard
When there are multiple dependencies, drawing and interpreting becomes difficult 

 

Ruling:
Certain Planning and Optimization techniques precisely require AOA network structure, and some might require AON format. It is hard to prefer one, yet opting AOA over AON and conversely, is solely based on specific project requirement. Nevertheless, the advantages of AON become more apparent and takes the upper hand over AOA.  

The two most common network diagramming techniques i.e. Activity on Arrow (AOA) & Activity on Node (AON) are used to showcase the graphical association among different tasks & are used to determine the critical path of a project. Using these two techniques we can draw those relationships between the different activities. Let us first understand the concept behind these two techniques. Both of these techniques comes under the purview of Program Evaluation Review Technique commonly known as PERT.

 

Activity on Arrow (AOA) uses arrows to represent the activities while the nodes are used to represent events, here the major emphasis is on the events or milestones. AOA only shows finish to start relationship. Below is an example to illustrate this:-

 

 

Here each activity is represented on an arrow i.e. A,B,C,D,E,F,G,H,I,J & each node represents the start & end of an activity i.e. nodes 1 & 2 represent the start & end of activity B, 1 & 3 for activity A & likewise for other activities we have nodes for start & finish i.e. 1,2,3,4,5,6,7,8,9.

 

Sometimes we also have to add dummy activities in order to showcase more complex relationships & dependencies amongst the activities which are represented by a dotted line arrow & has no duration associated with it.

 

Activity on Node (AON) as opposed to AOA represents the activities on the nodes & here the main focus is on the activities or tasks rather than events or milestones. Below is an example to illustrate this:-

 

 

As you can see each node in the diagram represents activities i.e. A,B,C,D,E,F,G & it has only two common event nodes which is the start & end node as opposed to the start & end event nodes for each activities in the AOA technique. Also in AON approach of network diagram creation we can showcase complex relationships without the need of using dummy activities. Also as opposed to AOA which only showcases finish to start relationship, here we can showcase any type of relationship i.e. start to finish, finish to start, start to start & finish to finish.

 

Due to its simplicity & compatibility with most of the project management software, AON is the preferable approach to create network diagrams. Let us now take an example & used both AOA & AON to create the network diagrams.

Let us see a project on setting up a hospital, below are the representation of activities & their sequence:-

 

Activity	Description	Predecessor Activities
A	Select administrative & medical staff	-
B	Select site & do site survey	-
C	Select equipment	A
D	Prepare final constructions plan & layout	B
E	Bring utilities to the site	B
F	Interview applicants & fill support staff positions	A
G	Purchase medical equipment	C
H	Construct the hospital	D
I	Develop an information system	A
J	Install the equipment	E,G,H
K	Train the support staff	F,I,J

 

Let us now observe the AOA & AON Network diagrams created for the above example.

 

AOA :

 

AON :

 

 

If we compare the above two figures, one can clearly see that the relationships amongst activities can easily be modelled with AON technique as there is no need for using the dummy activities particularly where we are creating network of activity K whose predecessors are F,I,J where you cannot straight away model this relationship in AOA approach as we need to use dummy activity between nodes 7 & 8 in order to project the activity arrow towards activity K which is not the case with AON approach as you can simply model the relationship by creating arrows from activity nodes F,I,J towards activity node K without the need for any dummy activity. 

 

Thus with AON approach we can easily showcase even complex relationships & thus this approach is more preferable compared to AOA. 

Activity on node (AON) method represents activity in a node & the arrows give the logical relationship between two nodes (activities) whereas Activity on arrow (AOA) represents an activity over the arrow & the nodes represent the events.

 

AOA can represent only finish to start dependency on the activities whereas AON can be used to represent finish to start, start to start, finish to finish and start to finish dependencies.

 

Also, there is no need for dummy activities in AON as compared to AOA. For example, in the below network diagram, activity 3 must start only after completion of activities 1 & 2. Similarly, activity 4 must start only after completion of activity 2. In AOA, a dummy activity "D" is introduced to complete the network diagram. For this reason, it is easier to construct/understand AON diagrams. 

 

Network Diagram : 

Network Diagrams mainly are 2 types - Activity on arrow (AoA) and activity on node (AoN) are being used to analyze various tasks involved for completing any project, especially when it is being used most to the time that is required to complete each task and the minimum amount of the time that is required to complete the entire project. It is well known method which comes under the Program Evaluation and Review Technique called as PERT. 

There are 3 stages involved in developing a network diagrams are; 

- Planning stage 

- Sequencing stage 

- Scheduling stage 

 

AoA - Activity on Arrow 

It use arrows to represent activities while nodes stand for events.

Each nodes contains 3 numerical values : 

* Start time 

* Finish Time 

* Float or Slack 

AoA diagrams show the finish to start relationships with a limited offer like Arrow represents the time span from the event at the start of the arrow to the event at the end. Activities that are represented as arrows have to be added to illustrate some of the more complicated relationships and dependencies that are present between the activities. 

Such diagrams used dummy activity to show the relationship among various dependencies and dummy activity need Zero time to complete it. 

AoN - Activity on Node

Each activity is represented by Arrow with nodes at each end to represent the Start and Finish of the activity. 

In AoN diagrams, the activity is placed on the node. The interconnection arrows would illustrate the dependencies that are there between the activities. They are more flexible and are capable of illustrating the main relationship types. Since the activity is on node the data usually can be placed on the activity. 

Such diagram usually start from Start Node and end with the End node. And generally dummy activity is not being used. 

We can easily understand the difference between AoA and AoN diagram with this example; 

 

So what should we use?

While there are some fundamental differences between AoA and AoN network diagrams, choosing one over the other is based on individual project requirements and some time matter of the personal choice. Some of the basic differences would be as follows.

AoA networks were more popular in the 1960s and 70s. 

AoN focus on Task while AoA focus on Events.

AoN diagrams are generally easier to Create as compared to AoA diagrams. 

AoA shows only finish to start relationship, while AoN can show any type of relationship like finish to start, finish to finish, start to start, start to finish. 

AoN is widely used network diagram technique and supported by most of the project management software's. 

 

There are two answers that have been selected as winners - Mohamed Asif and Chandra Shekhar Chauhan. Both the answers clearly highlight the difference between the two approaches using an example and have also mentioned valid reasons for selection of one over the other.