In project management, a dependency is what we call the relationship between two tasks when action or activity on one determines key dates for the other. Start and finish dates of the task are the focus of the relationship between a predecessor and successor activity with four types of logical relationships:
Finish to Start (FS): This is the most common dependent relationship and it occurs when the successor task cannot start until the predecessor task is finished. Examples of FS dependency might be that the building the walls must finish before the roof installation can start or when configuration can’t begin until the design is complete.
Start to Start (SS): These relationships occur when the successor task cannot start until the predecessor has started. This is done to optimize the project schedule and make efficient use of project resources. Examples of SS dependency might be that mortar is mixed at the same time tiling is started or if an external press release is sent out at the same time an internal announcement is made.
Finish to Finish (FF): These relationships occur when the successor task and the predecessor task need to finish at the same time, again to optimize the use of project resources. Examples of this might be finishing the plumbing and electric so that the inspector only comes once or it might be that configuration and testing complete at the same time to account for client feedback.
Start to Finish (SF): In this relationship, we see that the successor cannot finish until the predecessor has begun. An example might be that the inspection cannot be scheduled until a building permit is requested or Sign Off cannot be complete until UAT has begun.