A Precedence Diagramming Method (PDM), which is sometimes also known as the Activity on Node (AON) Diagramming Method, is a graphical representation technique, which shows the inter-dependencies among various project activities.
You may have also heard about another less commonly used technique in diagramming methods – the Activity on Arrow (AOA) diagramming method, which is a special case of the Precedence Diagramming Method.
In AOA, all dependencies are Finish to Start, and the duration is shown on arrows. That is why this diagramming method is known as the Activity on Arrow (AOA) diagram. PERT is an example of AOA diagram.
Activity on Arrow (AOA) diagram emphasizes milestones (events), and the PDM diagram emphasizes tasks.
The main benefit of Precedence Diagramming Method (PDM) is that it shows the activity dependencies, and it can be an important communication tool for stakeholders.
The Precedence Diagramming Method (PDM) consists of rectangles, known as nodes, and the project activities are shown in these boxes. These rectangular boxes are connected to each other with arrows to show the dependencies; therefore, these diagrams are also known as the Activity on Node (AON) diagrams.
The Precedence Diagramming Method uses four types of dependencies. Those dependencies are as follows:
1. Finish to Start (FS)
2. Finish to Finish (FF)
3. Start To Start (SS)
4. Start to Finish (SF)
Finish to Start (FS)
In this type of dependency, the second activity cannot be started until the first activity completes. This type of dependency is the most commonly used dependency in the diagramming techniques.
For example, to paint a wall you first need to build a wall. In this case, the first activity is building the wall and second activity will be painting. You can not start painting the wall unless the wall is ready.
Finish to Finish (FF)
Here, the second activity cannot be finished until the first activity finishes; in other words, both activities should finish simultaneously.
For example, let us say that you coding a program for a client, and the client is providing you the characteristics of the program. In this case you cannot finish coding for your program until the client gives you his complete requirements. Here, both activities should finish simultaneously.
Start to Start (SS)
Here, the second activity cannot be started until the first activity starts; both activities should start simultaneously.
Suppose you have to apply primary coating on the wall. To apply the coating, you also need to clean the wall. Therefore, one team will start cleaning the wall and second team will paint it. Both activities can be started at the same time.
Start to Finish (SF)
In this type of dependency, the second activity cannot be finished until the first activity starts.
For example, let us say you have to move into a new home, and your old home has to be demolished. In this case, you cannot move to your new home until it is ready. Hence, the second activity (construction of new home) must be finished before the first activity starts (you start moving into new home); i.e. if you are moving into your new home, you cannot start vacating your old home until the new house is completely ready.
Although this type of dependency is not commonly used, you as a project manager should know about the all types of dependencies. It is important for you, as the project manager, to understand all types of dependencies because it will help you considerably when drawing the network diagram and then creating the project schedule.
This concludes the precedence diagram method and the type of dependencies used in making the diagram.
In this blog post, I tried to give you the simplest examples of each type of dependencies used in project schedule network diagram; however, if you feel there could be any other, simpler, examples, then you can share in the comments section.
This topic is also very important for the PMP exam. Please understand these task dependencies for the precedence diagramming method and also remember the exhibits showing these dependencies. You may see one or two questions from this topic in your PMP certification exam.