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Strong dependencies

About strong dependencies

There are four standard types of dependencies, which are called 'Strong links' and have a scheduling impact. Every task on the timeline has two significant sides - to the left and to the right. These signify the Start and End Dates. Therefore, by linking two tasks (below referred to as Task A and Task B) we can expect four following combinations which are referred to as dependency types.

When one of the Auto-scheduling modes is enabled, such links will adjust the linked task's period. Strong links are displayed as a regular line and you can use them to indicate when a task should begin or end in relation to other tasks.

The default strong links are created during the installation:

In case there are multiple dependencies targeting one task, including ASAP dependencies, the latest possible date will be set for this task.

Strong dependencies

End to Start

When Task A finishes, then Task B starts and Task B can’t start until Task A is done.

Asap mode off: moving Task A to the right will also move Task B, as long as Task A's End Date is moved up to or ahead of Task B's Start Date. Moving Task A to the left has no effect on Task B. When Task B is moved to the left, it will not go further than Task A's End Date. Moving it to the right has no effect on Task A.
Asap mode on: moving Task A to the left or right will also move Task B, so that it starts on the first working day after A finishes. Moving Task B has no effect.

End to End

Asap off: when Task A ends, Task B also immediately ends. Task B can’t finish until Task A is done but they don’t have to end at the same time: Task B can end any time after Task A ends. The lag time determines the minimum gap. Moving Task A to the left changes nothing. However, if Task A is moved further than Task B's End Date, Task B will be moved in order to retain the dependency (and to end at the same time). Moving Task B to the left further than Task A's End Date will automatically move Task B's end at the same time as the end of Task A. Moving Task B to the right has no effect on Task A.

Asap on: when Task A ends, Task B also immediately ends. Task B can’t finish until Task A is done but they don’t have to end at the same time: if the lag time is set it determines the gap between the Task B end and Task A end. Moving Task A to the left or right will also move Task B so that the gap between the tasks is equal to the lag time set.

Start to End

Asap off: when Task A starts, 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. In this type of dependency, when Task A is moved to the right, if its Start Date exceeds the End Date of Task B, B will be also moved right. Moving Task A to the left changes nothing.

Asap on: when Task A starts, Task B can’t finish until Task A begins. Moving Task A to left or right moves the Task B so that it ends on the start date of Task A (+- lag time). Task B is not movable.

Start to Start

Asap off: this means that when Task A starts, Task B will also start. 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. When two tasks are connected this way and task A is moved to the left, task B remains unchanged. When Task A is moved right and exceeds the Start Date of Task B, then Task B will also be moved to the right in order to start at the same time as Task A. When Task B is moved to the left, it will never go further than the start date of Task A. If Task B is moved right, Task A remains unchanged.

Asap on: when Task A starts, Task B also immediately starts. Task B can’t start until Task A starts but they don’t have to start at the same time: if the lag time is set it determines the gap between the Task B start and Task A start. Moving Task A to the left or right will also move Task B so that the gap between the tasks is equal to the lag time set.

Scheduling mode vs strong dependencies

Strong dependencies do NOT affect auto bottom-up tasks (if the child tasks conflict with the change).

An auto bottom-up task can have children nested underneath it in the tree structure. In this situation task period is limited by the periods of its children (won't be affected by a strong dependency if there is a conflict):

Possible changes to make a strong dependency affect the tasks:

changing the scheduling mode of the parent to top-down:
drawing the Strong dependency to a child under the auto bottom-up parent (moving the child will expand the parent, but not move it).
Before:

After: