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What Are the Different Types of Task Dependencies

Once the WBS is built, you are ready to decide the order tasks will be performed. Some tasks depend on one or more tasks before they can start. For example, you can’t put up the walls until you pour the foundation. Some tasks need to start or finish at the same time, while other tasks can’t start until another one is already underway.

The predecessor and successor relationship you define provides the foundation for the critical path of your project. There are four different kinds of task dependencies:

Finish-to-Start Dependencies

The most common type of dependency is the finish-to-start relationship (FS). This relationship means that the first task, the predecessor, must be finished before the next task, the successor, can start. On the Gantt chart it is usually represented as follows:

 Finish to Start Dependency

Start-to-Start Dependencies

The next type of dependency is the start-to-start relationship (SS). This relationship means that the successor task cannot start until the predecessor task starts. On the Gantt chart, it is usually represented as follows:

 Start to Start Dependency

Finish-to-Finish Dependencies

The third type of dependency is the finish-to-finish relationship (FF). This relationship means that the successor task cannot finish until the predecessor task finishes. On the Gantt chart, it is usually represented as follows:

 Finish to Finish Dependency

Start-to-Finish Dependencies

The start-to-finish relationship (SF) is the least common task relationship and means that the successor cannot finish until the predecessor starts. On the Gantt chart, it is usually represented as follows:

Finish to Finish Dependency

Variations of Task Dependency Types

Of course tasks sometimes overlap – this is termed lead (or lead time). Tasks can also be delayed (for example, to wait while concrete dries) which is called lag (or lag time).

Task with Lead and Task with Lag Dependencies

Now that you know the different types of task dependencies, our next post will cover how to link tasks in Microsoft Project.