Activity on the Node (AON) is a project network diagramming tool that graphically represents all of the project activities and tasks, as well as their logical sequence and dependencies. Using AON, activities are represented as boxes (i.e. nodes) and arrows indicate precedence and flow. AON uses the activities defined in the WBS at its nodes. The time estimate for these activities may be obtained from estimation techniques like PERT or CPM.
PERT uses the project networking diagramming tools to create a visual representation of the scheduled activities that expresses both their logical sequence and interrelationships. PERT is a method to analyze the involved tasks in completing a given project, especially the time needed to complete each task, and to identify the minimum time needed to complete the total project. PERT also uses a statistical distribution that provides probability for estimating when the project and its associated activities will be completed. This probabilistic estimate is derived by using three estimates for each activity: optimistic, most likely, and pessimistic.
An optimistic estimate is the minimum time in which an activity or task can be completed. This is a best-case scenario where everything goes well and there is little or no chance of finishing earlier. A most likely estimate, as the name implies, is the normally expected time required to complete the task or activity. A pessimistic estimate is a worst- case scenario and indicates the maximum time the activity can take.
Thus, while AON is a network diagramming tool, PERT is a time estimation tool. Both tools are used to together to find the critical path and estimate the maximum time required to complete the project in project scheduling.