Arrow Diagramming Method (ADM)

Every project consists of a number of tasks which must be coordinated and scheduled to meet project goals and deadlines. A project planning network is a graphical representation of the overall project. Its graphical elements indicate activity duration and "precedence relationships," the order in which project tasks must be performed.

Various techniques exist for constructing network diagrams. One popular technique is the arrow diagramming method (ADM). The ADM, also known as the activity-on-arrow (AOA) network, uses arrows to represent activities and nodes to show dependencies.

In the ADM, activities are connected at points called nodes. A node preceding an activity arrow is the start event for that activity. A node following an activity arrow is the end event for that activity.

The ADM uses only finish-to-start (FS) activity dependencies. This means that activity A must finish before activity B can begin.

The first thing to remember when using the ADM is that an activity can't be represented by more than one arrow in the network diagram. Suppose activity A precedes two activities in a network. To avoid duplicating activity A on the diagram, you would need to follow one arrow representing activity A by an end node. Then follow this end node for A by the two successor activities, B and C.

The second thing to remember is that no two activities can have the same begin and end nodes. Instead, if two activities are related or dependent, you can use a "dummy" activity to show the relationship. A dummy uses no resources and is represented by a dashed arrow.

In addition to activities and dependencies, the ADM shows the early and late schedules for the project. Both the early time (TE) and the late time (TL) for an event appear on the event node.

The ADM also allows project managers to specify activity scheduling flexibility by calculating "float." Float is the amount of time available to complete an activity without affecting project duration. To determine a project activity's float, or slack, you would calculate the difference between the activity's late and early schedule times.

Once float has been determined for each event in the network, it is possible to find the critical path through that network. The critical path shows the earliest possible completion time of the entire project. To find the critical path in an AOA network diagram, you would follow the path that connects all events with zero float since activities with a positive float are, of necessity, not on the critical path.

The arrow diagramming method is a method used to construct network diagrams. Arrow diagramming is said to be "event-oriented" because the arrows represent activities. Arrow diagramming reveals the network sequence and timing of activities which is useful in managing the project's schedule.