Introduction:

Project dependencies, also called task dependencies are a key part of project planning.
Dependencies describe the relationships between tasks, and determine the sequence of work in a project.
They indicate when a task should begin or end in relation to other tasks.

Having a clear sequence of tasks helps you as a Project Manager to make the most out of your resources, avoid delays, and manage complex projects by identifying critical tasks, and prioritizing them.

For example, if you were to bake a cake, you would prepare the dough before you bake the cake, and you would finish baking the cake before you decorate it. If you do any of these tasks out of order, you will not be able to make your cake. This is true for any project. There are always things that need to be done before others, and managing them is a critical part of completing your project successfully.

Relationship types of task dependencies:

There are several different relationship types of dependencies in project management. Let us review the four main ones:

Finish-to-Start: Task B can only start when task A is finished

The Finish-to-Start dependency is the most common dependency and is generally the most straightforward.

For example, you have to finish baking the cake (task A) before you start decorating it (task B).

Start-to-Start: Task B can only start when Task A has started

With start-to-start dependencies, you can work on both tasks concurrently, but you need to start one of those tasks first.

For example, once the waiter has started taking the order (task A), the chef can start preparing the dishes (task B).

Finish-to-Finish: Task B can only finish when Task A has finished.

With Finish-to-Finish dependencies, you can work on both tasks simultaneously, but the second task is only able to complete after the first task is 100% completed.

For example, baking a cake and frosting it.
You can run both tasks (more or less) at the same time; you can bake the cake (task A) and make the frosting (task B) at the same time. However, the frosting of the cake (task B) can only be finished after baking it (task A).

Start-to-Finish: Task B can only finish when Task A has started

Start-to-Finish dependencies are less common in real life, and they are usually found in scheduling scenarios where there is some sort of handover between two teams.

For example, the morning shift (task B) in a restaurant kitchen cannot finish its day until the evening shift (task A) has arrived and started.

Lag / Lead-time in Dependency:

Lag time creates a delay between two tasks that share a dependency. For example, if you want two hours delay between taking the cake out of the oven (task A) and decorating it (task B) (because the cake must cool before decorating), create a finish-to-start dependency between the two tasks, and then add two hours of lag time.

The attached video illustrates how to create dependencies in Project Planning application: