Arrow diagramming method (ADM) is a network diagramming technique in which activities are represented by arrows. ADM is also known as the Activity-On-Arrow (AOA) method. It has been used long time to determine the critical path and identify resource problems and feasible solutions when the approximate duration and resource requirement of all the activities of network diagram are known. In this article we will analyze an arrow diagramming method example and review the basics of this method.
There are two main elements of the Arrow Diagramming Method(ADM) which are arrows and nodes. One arrow represents one activity to be performed. The tail of the arrow is the start of the activity, the head of the arrow is the end of the activity and the length of the arrow is the duration of the activity.
In the forward pass, we calculate the Start Dates of all the nodes. In the backward pass, the end node’s Start Date will be its Finish Date and the backward pass proceeds by subtracting the durations of the activities leading to the end node from the end node’s Finish Date to arrive at the Finish Dates for the preceding nodes.
The following example shows a simple Activity on Arrow Diagram.
In this example A,B,C,D,E,F,G rows are representing the activities. Numbers above the rows are activity durations. As shown in the schema above, in Arrow Diagramming Method(ADM), only F-S Relationship is used to link activities.
Step 1 : Forward Pass Calculation / Arrow Diagramming Method Example
Step 2 : Backward Pass Calculation/ Arrow Diagramming Method Example
Step 3 : Float Calculation for Each Activity / Arrow Diagramming Method Example
Float can be calculated by subtracting the Start Date of an activity from its Finish Date.
Step 4 : Identifying the critical path / Arrow Diagramming Method Example
The critical path is the longest path in the network diagram and total float of critical path is zero. As shown in the schema above, critical path is marked red.
The Arrow Diagramming Method(ADM) and The Precedence Diagramming Method(PDM) are two network scheduling techniques. The first one is the traditional one used widely in the past. The second one is the current method that we are using widely now. In this simpe arrow diagramming method example we show how to make forward, backward, total float and critical path calculation. Nowadays the arrow diagramming lost it’s popularity because of the introduction of software solutions but in order to use these sorftwares successfully, it is important to understand both scheduling techniques.
 CPM in Construction – A Manual for General Contractors (Copyright 1965 by the Associated General Contractors of America)