What are the four types of PDM relationships?

A Precedence Diagramming Method (PDM) or Activity on Node (AON) diagramming method is a graphical representation technique. It shows the interdependencies of activities and is used in schedule development. 

We use this method to draw the project schedule network diagrams, such as critical path network, critical chain network diagram, and others.

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

The Precedence Diagram (AON diagram) shows activity relationships, and it is an important communication tool for stakeholders.

The precedence diagram is made of rectangles known as nodes. These boxes show the project activities. An arrow connects two boxes and shows the relationship. Therefore, these diagrams are also known as activity on node (AON) diagrams.

Type of Dependencies in Precedence Diagram

The PDM uses four dependencies:

1. Mandatory Dependency
2. Discretionary Dependency
3. External Dependency
4. Internal Dependency

Mandatory Dependency

This dependency is also known as hard logic, meaning it’s a fixed reality that you cannot avoid. Starting the next activity depends on it.

For example, you cannot install the ceiling until you build all the walls.

Discretionary Dependency

This dependency is also known as preferential or soft logic, and it plays a role in optimizing resources.

For example, you can construct the four walls in any sequence. However, if constructing them in a certain sequence is beneficial, you build them in that order.

With discretionary dependency, you can change the sequence of activities as per your preferred logic.

External Dependency

The project management team has no control over an external dependency.

For example, you may need government approval before starting the next activity.

Internal Dependency

These dependencies are within the control of your project or organization.

For example, you cannot get a resource until it is free from another project.

Dependency Relationship in Precedence Diagram

The precedence diagramming method (PDM) uses four relationships:

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 relationship, the next activity cannot start until the first is complete. This is the most common relationship in the AON diagram.

For example, to paint a wall, you first have to construct it. With finish to start, the first activity is building the wall and the second activity is painting. You cannot start painting the wall until the wall is ready.

Finish to Finish (FF)

In this relationship, you cannot complete the next activity until the first is finished. Put simply; both activities should be finished simultaneously.

For example, let us say that you are coding a program for a client. The client gives you the features after completing a milestone. You cannot finish coding until you get the client’s complete requirements. 

Here, both activities should be finished simultaneously.

Start to Start (SS)

In this relationship, the next activity cannot be started until the first starts. Both activities should start simultaneously.

Example: To apply a coating on a wall, the wall must be cleaned.

Therefore, one team will clean the wall, and the second team will coat it. Both activities can start simultaneously.

Start to Finish (SF)

In this relationship, you cannot finish the next activity until the first starts.

For example, suppose you are moving into a new home, and your old home has to be demolished. In this case, you cannot move into your new home until it is ready. Hence, the second activity (construction of the new home) must be finished before the first activity starts (moving into a new home).

Put simply, when moving into your new home, you cannot start vacating your old home until the new house is ready.

Although this relationship is rare in an AON network, you must understand all the dependencies. It will help you draw the network diagram and develop the project schedule.

You may hear the term activity on arrow (AOA). This is a less commonly used technique in diagramming methods. The AOA method is a special case of the precedence diagramming method.

AOA diagram only uses the Finish to Start relationship. It shows the duration on the arrows; therefore, many experts call this diagramming method the activity on arrow diagram. PERT is an example of this technique.

There is a difference between the AON and AOA diagrams. The AOA diagram emphasizes milestones (events), and the AON diagram emphasizes tasks.

How to Draw a Precedence Diagram

To draw a precedence diagram, you will analyze your work and create a Work Breakdown Structure down to the activity level.

Then you will create a table, list all activities, and sequence them.

The next step is to add relationships to each activity. You will add what activity comes next.

Finally, you will draw the diagram.

Benefits of Precedence Diagramming Method

PDM offers many benefits to project management:

1. It helps you visualize relationships and dependencies among activities. This helps you plan and avoid risks. If any task is missing, you can easily identify it.
2. You can identify critical activities and focus on them. Any delay in critical activities will delay your schedule.
3. A project schedule network diagram is a good communication tool. Stakeholders can visualize activities and understand the schedule.
4. Without the Precedence Diagram, you cannot develop your project schedule.

Summary

The precedence diagram method has an important role in project management. Your project schedule depends on it, and it is a good communication tool. It is commonly referred to as AON, where nodes represent activities. AOA diagram is a special case precedence diagram where nodes represent milestones and duration is shown on the arrow.

This topic is important for the PMP exam. You may see one or two questions on this topic.

Here is where this post on Precedence Diagramming Method (PDM) or Activity on Node (AON) Method ends. 

Please share your experience using these diagramming techniques through the comments section.

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How many PDM relationships are there?

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