As part of project work, many activities need to be performed. However, you can’t perform these activities in any order you like, but you perform these activities in a logical sequence to obtain greatest efficiency given all the project constraints. “Sequence Activities” is the Project Management process where you create this logical sequence of activities. Now, there are various tools and techniques used for sequencing the project activities. Precedence diagramming method or PDM is a technique used for constructing a logical sequence of project activities.
In PDM technique, activities are represented by nodes, with all their logical relationships or dependencies, to show the logical sequence in which the activities are to be performed.
Activity-on-node (AON) is one method of representing a precedence diagram. This is the method used by most project management software packages. In this, nodes are denoted in “boxes”, and activity dependencies are denoted by “arrows”.
Predecessor Activity – It is the activity that logically comes before a dependent activity in a schedule. In the above figure, Activity A = Predecessor Activity
Successor Activity – It is a dependent activity that logically comes after another activity in a schedule. In the above figure, Activity B = Successor Activity
PDM Technique includes 4 types of dependencies
Finish to Start Dependency or FS Dependency
In a Finish-to-Start dependency, Successor activity cannot start until a predecessor activity has finished.
In this diagram, To Start Activity B, you have to Finish Activity A. i.e.. Finish A to Start B. (FS dependency).
Finish to Finish Dependency or FF Dependency
In a Finish-to-Finish dependency, the Successor activity cannot finish until a predecessor activity has finished.
In this diagram, To Finish Activity B, you have to first Finish Activity A. i.e.. Finish A to Finish B (FF Dependency)
Start to Start Dependency or SS Dependency
In a Start-to-Start dependency, Successor activity cannot start until a predecessor activity has started.
In this diagram, To Start Activity B, you have to first Start Activity A. i.e.. Start A to Start B (SS Dependency).
Start to Finish Dependency or SF Dependency
In a Start-to-Finish dependency, Successor activity cannot finish until a predecessor activity has started.
In this diagram, To Finish Activity B, you have to first Start Activity A. i.e… Start A to Finish B (SF Dependency).