The Precedence Diagram Method (also known as the activity on node method), is a visual presentation technique used to create a project network diagram. It is used along with the critical path method. Unlike the arrow diagramming method, activities are represented as nodes or boxes and linked by one or more dependencies. The dependencies show the order in which the activities will be performed.
Basically, while preparing a work schedule we use two common network diagrams:
• Arrow Diagramming Method or Activity on Arrow Method (ADM)
• Precedence Diagram Method or Activity on Node Method (PDM)
The Precedence Diagram Method (Activity on Node Method)
In Precedence Diagram Method (PDM), nodes or boxes are used to represent activities and arrows show the relationships. While creating a schedule by using this method, the activity dependencies must be determined. During this determination process, Predecessor and Successor activities must be specified.
Predecessor Activity: A predecessor activity is an activity whose start or finish controls start or finish of another activity
Successor Activity: A successor activity is an activity whose start or finish is controlled by the start or finish of another activity
Dependencies in the Precedence Diagram Method (PDM)
There are four types of relationships in the Precedence Diagram Method between the activities.
Finish to Start: A successor activity cannot start until a predecessor activity has finished
Start to Start: A successor activity cannot start until a predecessor activity has started
Finish to Finish: A successor activity cannot finish until a predecessor activity has finished
Start to Finish: A successor activity cannot finish until a predecessor activity has started
Finish to Start is the most common dependency in project network diagrams. Depending on the type of dependency a lead or lag may be used to represent the nature of activity relationships.
Lead is the time that an activity precedes the start or finish of its successor.
Lag is the time that an activity is delayed from the start or finish of its predecessor.
Network scheduling techniques are used in order to determine the project’s critical path and identify the relocation of resources to meet the project time management requirements. By the help of software, it is very easy to create the schedule network diagrams and analyze the critical paths effectively. Also, network diagramming software such as Primavera P6 and Microsoft Project Professional enables project managers, planning engineers, and schedulers to make updates and detailed analyzes quickly.