Three Ways to Diagram Projects

Before leaving on a trip you gas up the car and then check the road map for the best route. Project managers also have road maps that they can follow to choose the best route for their projects.

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.

In the precedence diagramming method, finish-to-start is the most commonly used type of dependency.

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.

Arrow Diagramming Method (ADM)

Every project consists of a number of tasks which must be coordinated and scheduled to meet project goals and deadlines. A project planning network is a graphical representation of the overall project. Its graphical elements indicate activity duration and "precedence relationships," the order in which project tasks must be performed.

Various techniques exist for constructing network diagrams. One popular technique is the arrow diagramming method (ADM). The ADM, also known as the activity-on-arrow (AOA) network, uses arrows to represent activities and nodes to show dependencies.

In the ADM, activities are connected at points called nodes. A node preceding an activity arrow is the start event for that activity. A node following an activity arrow is the end event for that activity.

The ADM uses only finish-to-start (FS) activity dependencies. This means that activity A must finish before activity B can begin.

The first thing to remember when using the ADM is that an activity can't be represented by more than one arrow in the network diagram. Suppose activity A precedes two activities in a network. To avoid duplicating activity A on the diagram, you would need to follow one arrow representing activity A by an end node. Then follow this end node for A by the two successor activities, B and C.

The second thing to remember is that no two activities can have the same begin and end nodes. Instead, if two activities are related or dependent, you can use a "dummy" activity to show the relationship. A dummy uses no resources and is represented by a dashed arrow.

In addition to activities and dependencies, the ADM shows the early and late schedules for the project. Both the early time (TE) and the late time (TL) for an event appear on the event node.

The ADM also allows project managers to specify activity scheduling flexibility by calculating "float." Float is the amount of time available to complete an activity without affecting project duration. To determine a project activity's float, or slack, you would calculate the difference between the activity's late and early schedule times.

Once float has been determined for each event in the network, it is possible to find the critical path through that network. The critical path shows the earliest possible completion time of the entire project. To find the critical path in an AOA network diagram, you would follow the path that connects all events with zero float since activities with a positive float are, of necessity, not on the critical path.

The arrow diagramming method is a method used to construct network diagrams. Arrow diagramming is said to be "event-oriented" because the arrows represent activities. Arrow diagramming reveals the network sequence and timing of activities which is useful in managing the project's schedule.