Activity on arrow

The original form of network diagram that has effectively been superseded by the precedence diagram format.

In an activity-on-arrow network, activities are represented by a line between two circles. The first circle represents the start of the activity and is known as the start event (sometimes called the i-node). The second circle represents the finish of the activity and is known as the finish event (sometimes called the j-node).


activity in an activity on arrow network


Space is allowed in the circles for calculation results from critical path analysis. The calculated times are the earliest event time (EET) and latest event time (LET). The earliest event time of the i-node is the earliest start of the activity and the latest event time is the latest start. Similarly, the earliest event time and latest event times of the j-node are the earliest and latest finishes of the activity.

The activity name is shown above the arrow and its duration is shown below. Its float is shown in brackets after the duration.

A network diagram is created by connecting activities according to their dependence upon each other. For instance, in the diagram below, activities 10-15 and 10-25 cannot start until 5-10 is finished. Activity 30-35 cannot start until both 20-30 and 25-30 are complete. Dotted lines are referred to as dummies. These simply indicate a dependency between two events and are not activities.


Activity on arrow diagram


Activity-on-arrow was in common use before the widespread use of computers for critical path analysis and so the drafting and calculations were performed manually. The convention of numbering the nodes at intervals of 5 was to allow additional nodes to be added while retaining an element of numerical sequence.


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