Sign in Subscribe

Explain the concept of project network diagrams.

Last updated on 


Project network diagrams are graphical representations that illustrate the flow and sequencing of tasks within a project. These diagrams help project managers and team members visualize the relationships among different project activities and understand the overall project schedule. There are two main types of project network diagrams: the Precedence Diagramming Method (PDM) and the Arrow Diagramming Method (ADM). Here, I'll explain the concept using the Precedence Diagramming Method.

  1. Nodes and Events:
    • Nodes: In a project network diagram, nodes represent events or milestones in the project. Events are points in time that signify the completion of one or more activities.
    • Activities: Activities are represented by arrows between nodes, indicating the flow of work from one event to another. Each activity has a duration, resources, and dependencies.
  2. Activity Sequencing:
    • Dependencies: Activities have dependencies, meaning that the start or completion of one activity may depend on another. There are four types of dependencies: Finish-to-Start (FS), Start-to-Start (SS), Finish-to-Finish (FF), and Start-to-Finish (SF).
    • Finish-to-Start (FS): The most common type, where the successor activity cannot start until the predecessor activity is completed.
  3. Precedence Diagramming Method (PDM):
    • Finish-to-Start (FS): In PDM, most activities use the Finish-to-Start relationship. The end of one activity triggers the start of the next.
    • Nodes with Activity Details: Nodes contain information about the activity, including the activity name, duration, and resources required.
  4. Critical Path:
    • Critical Path: The critical path is the longest path through the network diagram, representing the shortest time in which the project can be completed. Activities on the critical path have zero slack or float, meaning any delay in these activities will delay the entire project.
  5. Dummy Activities:
    • Dummy Activities: In some cases, dummy activities (represented by dashed lines) are used to indicate logical relationships between events without consuming time or resources. They are placeholders to show dependencies.
  6. Network Construction:
    • Forward Pass: Calculating the earliest start and finish times for each activity. The earliest start time for an activity is the maximum of the earliest finish times of its predecessors. The earliest finish time is the sum of the earliest start time and the activity duration.
    • Backward Pass: Calculating the latest start and finish times for each activity. The latest finish time for an activity is the minimum of the latest start times of its successors. The latest start time is the difference between the latest finish time and the activity duration.
  7. Float/Slack:
    • Float or Slack: The difference between the latest start time and the earliest start time (or latest finish time and earliest finish time) for an activity. Activities on the critical path have zero float, while non-critical activities have positive float.