Understanding Task Dependencies
Definition of Task Dependency
Task Dependency is a relationship in which a task or milestone relies on other tasks to be performed (completely or partially) before it can be performed. This is also referred to as a logical relationship. It is important to understand the types of task dependencies that can be used in Gantt Excel.
Setting dependencies between tasks is the most powerful feature of Gantt Excel. The in-built auto-scheduling automation of the template is able to update the dates of tasks based on changes that are made to its dependent tasks.
4 types of task dependencies are supported in Gantt Excel Ultimate
- Finish-to-Start (FS)
- Start-to-Start (SS)
- Finish-to-Finish (FF)
- Start-to-Finish (SF)
This is the most common type of dependency between tasks and a type that is used most often. This dependency means that if there are 2 tasks TASK A and B, then TASK A (FS) B means that, the dependent task (B) cannot begin until the task that it depends (A) on is complete… In other words, if TASK A is delayed by a day, then B will also be delayed and will begin a day later.
Example: If you are building the foundation for your house and you have two tasks, “Excavate” and “Pour concrete,” the “Pour concrete” task cannot begin until the “Excavate” task is complete
This dependency means that if there are 2 tasks TASK A and B, Task B can’t start until Task A starts. They don’t have to start at the same time: Task B can begin any time after Task A begins.
Example: To save time, you want to level concrete at one end of the foundation while it is still being poured at the other end. But Level concrete (B) can’t start until Pour concrete (A) has also started.
This dependency means that if there are 2 tasks TASK A and B, Task B can’t finish until Task A is completed. They don’t have to end at the same time: Task B can end anytime after Task A ends.
Example: Your team is adding the wiring to the building and inspecting it at the same time. Until Add wiring (A) gets done, you won’t be able to finish Inspect electrical (B).
Task B can’t finish until Task A begins. Task B can finish any time after Task A begins. This type of link is rarely used.
Example: The wooden window frames for your house are built off-site. You can’t finish Assemble windows (B) until Window frame delivery (A) begins.
Lag time, a time interval can be set between dependent tasks to allow for required delays. For example, in a construction project, you need to let the foundations set before building the walls. In this case, lag time should be set for concrete to set. A successor task can only start once the defined lag time (specified in days) has elapsed.
Predecessor and Successor Tasks – Definition
Predecessor and Successor are two types of dependent tasks that belong to one and the same project or process and that are performed under dependency rules. A predecessor task determines the start or finish date of its successor task.
Both types are available in any work breakdown structure that includes dependent activities. Dependency rules between tasks determine which of the tasks are predecessors and which ones are successors. Graphically it can be presented as a tree-like structure in which every higher level is the successor of its lower level, and vice-versa – every lower level is the predecessor of its higher level.
Essentially, the dependency between tasks and sub-tasks is the predecessor-successor relationships. Such relationships are used in various project management methodologies.