Critical Activities

An activity is said to be critical if its duration cannot be extended without delaying the completion of the project.

Notice that there are three paths through the network:

The duration of the project is determined by the longest path - D cannot begin until A has been completed.  The critical activities are those which lie on a longest path through the network - here A and D are critical.  Any extension or delay in a critical activity will make such a path longer still, thereby delaying completion of the project. A longest path through a network is called a critical path.  On the diagram the critical path is shown in red.