What is a Project Network Diagram?

A project network diagram, also known as a precedence diagram, is a handmade or software-created diagram that shows the relationships in time and dependency of steps needed to complete a project. The diagram clarifies which steps can precede others, which steps must succeed others, and which can occur simultaneously, as well as other project constraints. It also shows when lead time allows beginning one task before another is complete as well as when lag time is needed after a step has been completed before a succeeding step can commence. A project network diagram is helpful in computing start and end dates, apportioning resources and personnel, and analyzing scheduling choices.

There are a variety of ways to create a graphical representation of a project network diagram. It is common to use left to right progression to show change from earlier time, at the left, to later time, at the right. It is also common to prepare for creating a project network diagram by preparing a chart of activities, roughly in chronological order, with verbal notes about dependencies.

One type of graphical representation is the precedence diagramming method (PDM), also known as the activity-on-the-node method (AON). This method uses a rectangle to represent each activity and arrows to represent relationships. When there are multiple predecessors or multiple successors or several lines of development going on simultaneously, this is shown in the diagram. The PDM also shows Finish to Start dependencies (FS), where one activity must complete before another begins; Start to Start dependencies (SS), where one activity may start only when another activity has commenced; Start to Finish dependencies (SF), where one activity cannot be completed prior to another activity starting; and Finish to Finish dependencies (FF), where one activity cannot be completed prior to another activity completing.

With a project network diagram, one is able to compute ES (Early Start), EF (Early Finish), LS (Late Start), and LF (Late Finish date) and the critical path. The ES is the earliest start time for an uncompleted project or part of a project, while the EF is the earliest date at which an uncompleted project or portion of a project can finish. The LS is the latest date at which a project or project step can begin without causing a delay to some important milestone, usually the finish date, while the LF is the latest date at which a project or project step can finish without causing a delay to some important milestone, either the beginning of another step or the finish date. The critical path is the activity sequence by which the earliest completion date for a project is determined.