Types of project network diagrams: Arrow diagram and precedence diagram
There are two main types of network diagrams in project management: the arrow diagramming method (ADM), also known as “activity network diagram” or “activity on arrow”; and the precedence diagramming method (PDM), also known as “node network” or “activity on node.”
Arrow diagram method (ADM)
The ADM, or activity network diagram, uses arrows to represent activities associated with the project. It’s important to note that, due to the ADM’s limitations, it is no longer widely used in project management. However, it’s still useful to understand ADMs, so that you can recognize these diagrams if they arise in your work environment.
The tail of the arrow represents the start of the activity and the head represents the finish.
The length of the arrow typically denotes the duration of the activity.
Each arrow connects two boxes, known as “nodes.” The nodes are used to represent the start or end of an activity in a sequence. The starting node of an activity is sometimes called the “i-node,” with the final node of a sequence sometimes called the “j-node.”
The only relationship between the nodes and activity that an ADM chart can represent is “finish to start” or FS.
Occasionally, “dummy activities”—arrows that do not represent a direct relationship—need to be included in ADM network diagrams. In the diagram below, activity C can only occur once activities A and B are complete; in the network diagram, you’ve connected activity A to activity C. Perhaps we’re talking about tiling a floor (activity C): It can only begin once the concrete is poured (activity A) and the permits are obtained (activity B). Since activities A and B are not directly related—A doesn’t lead to B, and B doesn’t lead to A—you’ll need to draw a dummy activity between B and C to show that C is dependent on B being completed.An ADM chart also does not have a way to encapsulate lead and lag times without introducing new nodes and activities.
Precedence diagram method (PDM)
PDM network diagrams are frequently used in project management today and are a more efficient alternative to ADMs. In the precedence diagramming method for creating network diagrams, each box, or node, represents an activity—with the arrows representing relationships between the different activities. The arrows can therefore represent all four possible relationships:
“Finish to Start” (FS): When an activity cannot start before another activity finishes
“Start to Start” (SS): When two activities are able to start simultaneously
“Finish to Finish” (FF): When two tasks need to finish together
“Start to Finish” (SF): This is an uncommon dependency and only used when one activity cannot finish until another activity starts
In PDM, lead times and lag times can be written alongside the arrows. If a particular activity is going to require 10 days to elapse until the next activity can occur, for example, you can simply write “10 days” over the arrow representing the relationship between the connected nodes.