Network Diagrams

A Network Diagram is a b nothing but a visual representation of a project schedule.  It helps in project planning and controlling the project from start to end. It helps to find a critical path, critical activities, free float, total float and much more on a project schedule.

Types of Network Diagrams | ADM & PDM

We have two types of network diagrams namely;

  • The arrow diagram method – ADM
  • The precedence diagram method – PDM | AON | AOA

Arrow Diagram Method – ADM

In ADM, arrows represent the project activities like a tail is for start of activity and head for the finish. the arrow length represents the duration for that activity.

How to draw a network diagram through Arrow Diagram Method - ADM These arrows of all activities will connect through nodes and we will get a network diagram. 

PDM | AON | AOA

PDM is also called Activity on a node – AON and Activity on Arrow – AOA is a special type of PDM.

In PDM we have relationships or the dependencies that help to make reports for different project stakeholders. Like, if a contractor has to wait for another contractor to finish the job first then can start on. This makes life easier for a project manager.

The Activity on Arrow (AOA) diagramming method has all the activities with only one relationship that is FS- Finish to Start. In the Program evaluation review technique – PERT we use AOA diagrams.

Below is a simple comparison between AOA and AON for easy understanding. Make sure the circle may be rectangles, square or any proper consistent shape.

AOA & AON Comparison

AOA diagram normally good to show milestones or events as duration is zero, and the AON are better to represent task in a network diagram.

Hope all these terminologies a crystal clear now. Let’s see PDM in more details.

Types of Relationship | PDM

As I said earlier, the arrows in PDM represent the relationships between the activities. That relationship can be one of the following:

  1. Finish to Start – FS
  2. Start to Start – SS
  3. Finish to Finish – FF
  4. Start to Finish – SF

Let us dig more into these with easy examples.

Finish to Start – FS

In this type of relationship where the next activity (successor) cannot be started until the first one (predecessor ) completes. This one is the most of commonly used in PDM. More than 75% of relationships in a schedule are Finish to Start. They also call it a natural relationship.

Finish to Start Relationship in Network Diagrams

Example: Floor tiling can not be started until the Waterproofing is done. Or let see another example, A slab work cannot be started before completing the column work.

Start to Start – SS

In this type of relationship where the next activity (successor) first one (predecessor ) has to be started at the same time. Start to start a relationship is helpful during crashing a schedule. Some people may be thinking that a  SS Relationship is a Must Start Together. So, if one of the activities Starts other one should automatically start.

Start to Start Relationship in Network Diagrams

Example: If Paintworks is started on one floor of a tower then you can start paintwork on the other floor if ready. This will save time in your schedule. But most of the time as I said SS relationship is the best for fast-tracking.

In another way, let say a floor is released from another contractor and is ready for your paintwork then it is a must start to start type relationship.

Finish to Finish – FF

In this type of relationship where the next activity (successor) first one (predecessor ) are completed at the same time. Normally, this relationship is used for connecting to a finish milestone.

Finish to Finish Relationship in Network Diagrams

Example: You are asked to finish a concrete job at floor 10 and client put is as a milestone on your schedule. Now, once you will complete the last activity like poring on concrete and this milestone will be automatically completed.

Start to Finish – SF

In this type of relationship where the first one (predecessor ) cannot be started until the successor is completed. Make sense!

This one is a highly undesirable relationship in any schedule. But is being used in some schedules.

Start to Finish Relationship in Network Diagrams

Examples: It is really hard to find examples of this particular relationship. Let’ see below one and try to understand the stuff.

  1. A security guard cannot start his duty unless the other one will finish. To be more specific, the start of a new shift is a signal to finish the shift of the current security guard.
  2. In any shift work, the reliever has to arrive, clock in and start his shift before the extant shift worker can clock off and end his shift. But if the reliever is late, the existing shift can’t finish. It can only finish once the following task has actually started.

I hope, you are quite clear with these examples. If you can add more examples then please mention in comment.

Conditions for a Network Diagram

These conditions must fulfill to develop a network diagram;

  • Activities should be clearly defined for the complete scope of a project
  • Logic must be there among activities
  • Time estimation of all activities should be done already

Benefits of Project Network Diagrams

  • Helps to estimate time accurately
  • Helps to make proper decisions
  • Helps to find opportunities for optimization

Wrap Up

AOA, AON, PDM, and ADM term will not bother your head anymore. It was a tough ask for me when I was preparing for my PMP exam. Network diagram makes things easy for a project team as it clearly visualized the things for them.


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