Activity Network Diagram (AND): Definition & Example | Study.com

AND Symbols

As a version of an Arrow diagram, arrows connect activities to indicate both sequence and dependency. Because the purpose of an AND is to identify the critical path through a project along with its time requirements, a basic AND doesn’t require many symbols. In fact, many ANDs are drawn using only three or four drawing symbols, much like a flowchart. As shown in Figure 1, a rectangle with rounded corners represents an activity, an arrow connecting one activity to another shows sequence, a diamond shaped symbol indicates a binary condition test (true or false, yes or no, etc.), and a rectangular box following and activity indicates a product, result, or data that is being passed to the next activity.

Figure 1. The primary symbols used in activity network diagrams (AND)

Other symbols, primarily those of the Unified Modeling Language (UML) or the CPM symbol sets are often included on an AND to provide for a higher level of detail or specification for the activities and their scheduling. There are several diagramming software tools available, each of which typically defines its own version of activity diagrams and the symbol set associated with it.

Creating an AND

Let’s say you are tasked with creating an AND for a software application project. The AND is to be used to identify the critical path of the project. Typically, a work breakdown structure (WBS) will have preceded the AND and the project’s activities, in either summary or detail, will have been identified. In this case, the activities identified are those common to the System Development Life Cycle (SDLC), which are requirements gathering, requirements analysis, system and user interface design, system development, system testing, and implementation. While each of these activities is typically a major sub-project, for this example, let’s assume they are your project’s activities. Figure 2 illustrates the sequencing and dependencies of the project’s activities.

Figure 2. An example of an simplified activity network diagram (AND)

The AND shown in Figure 2 is a simplified example of a software development project and its activities. Notice that the activities of this project are mostly in series. This means that an activity must be completed before the next activity can start. However, following Activity C an approval of the system design must be obtained from the stakeholders and project sponsors before Activities D, E, and F can begin. The bar symbol inserted after the approval decision indicates that stakeholder approval directly affects all three of the activities connected to it. After the approval is granted, Activities D, E, and F can all be started at the same time or in parallel.

In addition to its name, each activity symbol could contain other information. In our example, an estimated number of days is given for each activity. However, the information included for the activities could be no more than our example or more detailed information, such as early and late start and end dates, duration in days, or perhaps the program classes, files, or documents associated with each activity, like those found on a UML diagram. The data included on an AND is really up to the project manager or stakeholders and may be any data deemed relevant.

The AND in Figure 2 contains a single number for each of the activities of this project. This number represents the expected duration in days for each activity. However, in this simplified example, you wouldn’t know if this time is realistic, optimistic, or pessimistic. It may even be the average time for this type of activity based on past experience.

Figure 3. A PERT data block can be used in an AND.

Figure 3 shows a PERT (Program Evaluation Review Technique) diagram data block that is filled in for each activity. This level of detail may or may not be needed for every project, but having this data available on an AND can help you to determine the critical path through a project and the duration of four different estimates:

• Most likely time: typically, this time is equal to the duration of the critical path because the activities expected to take the most time are included.
• Optimistic time: this is the best possible (shortest) time for completing the project.
• Pessimistic time: the opposite of the optimistic time, this the longest time the project may take based on its estimates.
• Expected time: this time is calculated using the other time estimates. It is calculated by adding the Optimistic time, four times the Most likely time, and the Pessimistic time and dividing this sum by six. For example, if the Optimistic time for a project is 64 days, four times the Most likely time of 70 days is 280 days, and the Pessimistic time is 84 days, the Expected time calculates to a bit more than 71 days (i.e., 64 + 280 + 84 = 425 / 6 = 71.33).

Lesson Summary

An Activity Network Diagram (AND) is a graphical representation of a project’s activities that depicts the serial and parallel relationship between activities using blocks and arrows. ANDs are commonly used in project management to identify a project’s critical path and estimates of a project’s expected completion time.