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.

Precedence Diagramming Method (PDM)

A project is a series of interconnected activities that can be represented by the project network diagram. Various techniques exist for constructing network diagrams. One of the most common is the precedence diagramming method (PDM).

The PDM uses nodes to represent project activities and connecting arrows to show activity dependencies. This technique is also called the activity-on-node (AON) network approach.

To construct a network diagram, you need to consider four types of precedence relationships: finish-to-start (FS), start-to-start (SS), start-to-finish (SF), and finish-to-finish (FF).

• finish-to-start (FS)
  The FS relationship is one in which activity A must finish before activity B can begin. This is the most commonly used type of precedence relationship.
  

• start-to-start (SS)
  The SS relationship is one in which activity A must start before activity B can start. As an example, let activity A be compiling a phone list, and Activity B be calling the people on the list. In this example, compiling the phone list (A) must start before calling the people on the list (B) can start.
  

• start-to-finish (SF)
  The SF relationship is one in which activity A must start before activity B can finish. Let activity A be breathing on one's own. Let activity B be breathing with the aid of a respirator. Breathing on one's own must start (A) before breathing with the aid of a respirator (B) can finish.
  

• finish-to-finish (FF)
  The FF relationship is one in which activity A must finish before activity B can finish. Let activity A be a telemarketing department compiling a phone list. Let activity B be calling the people on the list. Compiling the phone list (A) must finish before calling the people on the list (B) can finish.
  

To convey as much information as clearly as possible, project network diagrams also include early and late schedules.

• the early schedule
  The early schedule is made up of the earliest start (ES) and earliest finish (EF) times. In a project, the ES for an activity is the earliest possible time that the activity can begin. The EF is the earliest possible time that the activity can end.
  
  You can determine the ES for an activity by looking at its predecessors. No predecessors means that the ES is day one, the beginning of the project. One predecessor means that the ES for the activity is dependent on the EF of the predecessor. More than one predecessor means that the ES of the activity depends on the maximum of the EF times of the predecessor activities.
  

• the late schedule
  The late schedule consists of the latest start (LS) and latest finish (LF) times. In a project, the LS and LF are the latest times that an activity can begin and end without causing project delay.
  
  You can calculate the LS and the LF times by using the early schedule. First, set the LF time of the last activity to the EF time of the same activity. Likewise, you can determine the LF time of all immediate predecessor activities by choosing the maximum of the EF times on all predecessor nodes.
  

Once the early and late schedules are in place, you can determine a float for an activity by taking the difference, LF minus EF.

LF - EF = LS - ES

The activity has no float if this number is zero. Float allows for some flexibility in project management and can change as the project progresses.
In a project network diagram, the series of activities that determines the earliest possible completion of the project is called the critical path. The critical path is usually defined as those activities for which float is zero.

To determine the critical path, begin with the first activity in the project. Look at its successors. Compare the successors' float values. Select the one with zero float. This is the second activity on the critical path.

Continue from the second activity on the critical path and compare the float for its successors. Select the activity that has zero float and include it in the critical path.

Continue this process to the final activity for a complete critical path. Ultimately, a project cannot finish any sooner than the time it takes to complete the activities on the critical path.

The PDM represents a project as a network of interrelated activities. The PDM is a way to prepare project activities for scheduling by organizing them in terms of the project's relationships, early and late schedules, float, and critical path. In this way, the PDM allows you to arrive at a realistic, workable project schedule.