Constructing a Project Network Diagram

In this article, we’ll illustrate how to construct a Precedence Diagram. The steps are:

1. Specify predecessors and successors, given an activity list.
2. Arrange the activities in order of execution.

While creating an activity list, you need to ask yourself the following questions:

• What activities happen before the activity being examined?
• What activities can happen at the same time with this activity?
• What activities happen after this activity?

For example, suppose you are constructing a shed and the following activity list has been defined:

Purchase Wood; Hire Workers; Dig Foundation; Lay Foundation; Harden Foundation Cement; Purchase Plot; Select Design; Assemble Shed; Purchase Paint

Suppose, you are examining the Purchase Wood activity. Ask yourself the three questions list above.

• What activities happen before the activities being examined? Some activities that may occur before the “Purchase Wood” activity include: Purchase Plot and Select Design
• What activities can happen at the same time with this activity? Some activities that may occur at the same time as “Purchase Wood” activity include: Hire Workers and Purchase Paint
• What activities happen after this activity? Some activities that may occur after the “Purchase Wood” activity include: Assemble Shed.

From the answers, we have determined the predecessors and successors of the Purchase Wood activity. Similarly, you can examine each activity and develop relationships between each one (see the image below - click for a larger view). You will then have table of activities, which also contain information about predecessors and successors.

After creating the table, you can then proceed to constructing the Precedence Diagram. An example precedence diagram is shown in the image. Refer to the table, while creating the diagram. The visual representation helps you to identify missing activities. In this example, “Cut Wood” may be a missing activity, that’ll depend on “Hire Workers.” Can you spot any other missing activities?

Usually, there are multiple paths of completion for any project, hence the Precedence Diagram may vary. For example, in the example shown in the image, we could make Purchase Paint dependent on Purchasing Wood. This would alter the diagram and consequent project schedule.

In a complex project, you will probably not create the Precedence Diagram manually as we have done in this example. You will most likely use software, such as PS8, to create the Precedence Diagram. However, even in the software, you will need to manually enter the activities list and the dependencies between them. The software you use, such as PS8 or Microsoft Project, will consist of a gamut of other useful features, like automatically creating the critical path and a Gantt chart.

Another approach to creating a Precedence Diagram used often in teams involves writing activities on stickies and then sticking them on a whiteboard. You will then manually draw the relationship between the activities. The advantages of using this approach are:

• The Network Diagram can be created with inputs from several people at the same time.
• High visibility thanks to the whiteboards.
• Quick modification because stickies can be placed anywhere and arrows can be erased easily.

After creating a Network Diagram, you will need to use the Critical Path Method (CPM) to determine:

• the critical activities (activities that can cause delay),
• the Float for each activity,
• the optimal project activity flow, and
• create a schedule.

For more information, see the Using the Critical Path Method article.