The project network diagram, also known as a project manager's road map, is one of the inputs to schedule development. It is a schematic display of the project's activities and their logical relationships or dependencies. It may be produced manually or on a computer, and may include full project details, or have one or more summary activities. The diagram should be accompanied by a summary narrative that describes the sequencing approach.
Project managers use three principal types of network diagrams: precedence diagramming method (PDM), arrow diagramming method (ADM), and conditional diagramming method (CDM).
Precedence diagramming method (PDM)
The precedence diagramming method (PDM) uses nodes to represent activities. Arrows join the nodes together and indicate the dependencies between activities. This technique is also known as activity-on-node (AON) and is the method most widely used by project management software.
The precedence diagramming method is based on four types of dependencies: finsh-to-start, finish-to-finish, start-to-start, and start-to-finish. The first activity in a dependency relationship is referred to as the "from" activity. The second is referred to as the "to" activity.
- In a finish-to-start dependency the "from" activity must finish before the "to" activity can start. For example, on a courseware development project, you must finish the scripting before the graphics can be developed.
- In a finish-to-finish dependency, the "from" activity must finish before the "to" activity can finish. For example, car body and engine production can be started at the same time. The last step in the engine production phase is to install it in the body. Therefore, the body must be finished before the engine can be finished.
- In a start-to-start dependency, the "from" activity must start before the "to" activity can start. For example, on a telemarketing project the compilation of phone lists must be started before people can actually be called.
- Finally, in a start-to-finish dependency, the "from" activity must start before the "to" activity can finish. For example, if your car refuses to start, you may need to jump start the battery with booster cables. The engine must start before you can finish jump starting the car.
Arrow diagramming method (ADM)
The arrow diagramming method (ADM) uses arrows to represent the activities and connects them at nodes to show dependencies. This technique is also known as activity-on-arrow (AOA). Although less common than the PDM, it is still the technique of choice in some application areas.
In an ADM, "dummy activities" are used to show logical relationships when logical relationships cannot be completely or correctly described with regular activity arrows. A dummy activity uses no resources, has a duration of zero, and is represented by a dashed arrow.
Conditional diagramming method (CDM)
The conditional diagramming method (CDM) allows you to diagram activities that must be repeated more than once. This technique also allows you to diagram non-sequential activities. The two most widely used techniques for creating a CDM are graphical evaluation review technique (GERT) and system dynamics.
Activities that must be repeated more than once are known as loops and can affect the project schedule if their durations are not calculated properly. An example of a loop may be the testing component of a project that needs to be repeated more than once.
When your project has an activity that only occurs under the right conditions, you will need to add conditional branches to the schedule. For example, a conditional branch may be added following an inspection activity. This would indicate that if errors are detected in the product, changes to the product's design may be needed.
Project managers use standardized network diagrams to create project network diagrams faster than they could by drawing them out using a pen and paper. These networks can include an entire project or only a portion of it. Portions of a network are commonly referred to as subnets or fragnets. Subnets are especially useful when a project has several identical or near identical features. Examples of subnets include constructing floors in a high-rise office building, or doing clinical trials on a pharmaceutical project.
Project network diagrams can be used as a guide for your project team and to help your management team better monitor project progress. This ultimately increases your chances of executing a successful project.
