A project network diagram shows the time relationships and dependencies of steps in a project. It can be created in various ways, including the precedence diagram method (PDM). The diagram allows for the calculation of start and finish dates, resource allocation, and analysis of scheduling decisions. It also calculates ES, EF, LS, LF, and the critical path.
A project network diagram, also known as a precedence diagram, is a hand-made or software-created diagram that shows the time relationships and dependency of the steps required to complete a project. The diagram clarifies which steps can precede others, which must happen to others and which can occur simultaneously, as well as other design constraints. It also shows when lead time allows one activity to start before another is completed, and when there is a delay time after a step is completed before a next step can start. A project network diagram is useful for calculating start and finish dates, allocating resources and staff, and analyzing scheduling decisions.
There are several ways to create a graphical representation of a project network diagram. It is common to use the left-to-right progression to show the change from the previous tempo, on the left, to the next tempo, on the right. It is also common to prepare for creating a project network diagram by preparing a graph of activities, in roughly chronological order, with verbal notes about dependencies.
One type of graphical representation is the precedence diagram method (PDM), also known as the node activity method (AON). This method uses a rectangle to represent each activity and arrows to represent the relationships. When there are multiple predecessors or multiple successors or multiple lines of development going on at the same time, this is shown in the diagram. The PDM also shows End-to-Start (FS) dependencies, where one activity must complete before another begins; Start to Start (SS) dependencies, where a task can only start when another task has started; Start-to-finish (SF) dependencies, where one activity cannot be completed before another activity begins; and finish-to-finish (FF) dependencies, where one task cannot be completed before another task has completed.
With a project network diagram, ES (Early Start), EF (Early Finish), LS (Late Start) and LF (Late Finish date) and critical path can be calculated. ES is the earliest start time for an incomplete project or part of a project, while EF is the earliest date an incomplete project or part of a project can finish. LS is the latest date that a project or project phase can begin without causing a delay to some important milestone, usually the finish date, while LF is the latest date that a project or project phase project can end without delaying a major milestone, the start of another step, or the finish date. The critical path is the sequence of activities by which the earliest completion date of a project is determined.
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