After defining the activities, the next step in the process of developing a project schedule is to sequence the activities. Sequencing the activities means to put them in the order in which they will occur and to also indicate the relationships between them. The best way to sequence activities is to use the precedence diagramming method (PDM), which is a graphical representation of activities in a project, represented on nodes, with the relationships between them indicated by arrows. It is more commonly called the activity-on-node (AON) diagram because the information about the activities is contained with them the rectangular node, and the relationship between individual nodes is represented by arrows. The arrows show a predecessor and successor relationship between activities. An activity can be a predecessor of other activities, meaning that it must be done before them. The same activity can also be a successor activity to one or more activities, meaning it must be done after them. There are four types of relationships that exist between predecessor and successor activities:
- Finish-to-start (FS) A finish-to-start relationship is one in which the successor activity cannot start until the predecessor activity has finished.
- Finish-to-finish (FF) A finish-to-finish relationship is one in which the successor activity cannot finish until the predecessor activity has finished.
- Start-to-start (SS) A start-to-start relationship is one in which the successor activity cannot start until the predecessor activity starts.
- Start-to-finish (SF) A start-to-finish relationship indicates that the successor cannot finish until predecessor starts.