The Precedence Diagramming Method (PDM) is a tool for scheduling and sequencing events in a project plan based on their relationships with other events. The model allows you to visually map activities and their dependencies. Within this model there are four types of dependencies or logical relationships that are possible: finish-to-start (FS), finish-to-finish (FF), start-to-start (SS), and start-to-finish (SF). The start-to-finish relationship seems to be the model that causes the most confusion for students of the PMP exam, perhaps because there are not many good examples of this relationship. Start-to-finish occurs when “the completion of the successor activity depends on the initiation of the predecessor activity,” or, said so that normal folks can process it: Activity A must start before Activity B can finish.
Think about how you would implement a new system and turn an old system off. You can complete (i.e., turn off) the old system as soon as the new system has started in production—but not before then. Let’s try an example or two and see if it clicks:
Say you’re building a new gas pipeline. You would first finish construction and implementation of the new pipeline before you would begin shutting down and breaking down the old pipeline. Similarly, implementation of a new accounting system has to be completed before you can begin turning off the old system. Make sense?
Don’t spend too much time sweating over this particular model, however. Even though it’s not a difficult concept, the start-to-finish relationship is rarely used.