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Task Scheduling Relationships

Work planners can create relationships between tasks that determine how the tasks are scheduled. For example, if you have three tasks on a work order, you may want to set up the work order so that task C is not scheduled to start until tasks A and B are finished. You can use task relationships to enter this type of scheduling information.

This section explains how task relationships work and the types of relationships that can be created.

Task Relationships

Task relationships allow you to link the start or finish time of one task to that of another task or tasks. This information is used when creating schedule entries for the tasks.

A task relationship links a predecessor task with a successor task. A predecessor is a task that must start or finish before another task can start or finish. A successor is a task that cannot start or finish until another task starts or finishes. For example, if you create a finish-to-start relationship between task A and task B, task A is the predecessor and task B is the successor.

Four relationship types are available:

Relationship Type	Description	Example
Finish-to-start (FS)	The successor task cannot start until the predecessor is finished. This is the default relationship type for tasks on a work order.
Start-to-start (SS)	The successor task cannot start until the predecessor starts.
Finish-to-finish (FF)	The successor task finishes at the same time as the predecessor.
Start-to-finish (SF)	The successor task finishes at the same time as the predecessor starts.

Note: Task scheduling relationships (predecessors and successors) do not apply when simple scheduling is used.

Default Relationships on Multi-task Work Orders

When you add tasks to a work order, the system automatically enters finish-to-start relationships between the tasks. For example, task B automatically has a FS relationship with task A. If you change the order of the tasks, the system adjusts the relationships to reflect the new order. You can delete or modify the default FS relationship, as necessary.

Lags and Overlaps

For each task relationship, you can enter a lag time. You can use lags to offset the start or completion date of two linked tasks. For example, if two tasks have a finish-to-start relationship with a four-hour lag, task B will start four hours after the completion time of task A.

You can also use lag time to overlap two tasks by entering a negative lag time. For example, to set task B to start four hours before task A is completed, link the tasks with a finish-to-start relationship, and enter a lag of “-4” hours.

The following table shows some examples of how lags and overlaps affect each relationship type.

Relationship Type	Lag/Overlap	Example
Finish-to-start	4 hours
	-4 hours
Start-to-start	4 hours
	-4 hours
Finish-to-finish	4 hours
	-4 hours
Start-to-finish	4 hours
	-4 hours

Note: You can also set lag time on individual trade, service, or tool requirements for a task. See Lag Time on Resource Demands.

When are the Start and Completion Dates Calculated?

When task relationships have been defined for a task, APM re-calculates the task’s planned start and completion dates when:

•	The planned start or completion dates change on one of the predecessor tasks

•	The task’s duration changes

•	A relationship is deleted

•	A predecessor task is cancelled

Which Dates are Used to Calculate the Successor’s Dates?

The date used to calculate the successor’s start or completion dates varies depending on the type of relationship. For example, a FS relationship uses the predecessor’s completion date so the successor cannot start until the predecessor is completed. While the predecessor is in planning, its planned completion date is used. When the predecessor is scheduled, its scheduled completion date and time are used. Finally, when the predecessor is completed or closed the actual completion or close date is used.

The relationship type determines which of the predecessor’s dates are used to update which of the successor’s dates:

Relationship Type	Predecessor Date Used	Successor Date Set
Finish-to-start	Completion date	Start date
Start-to-start	Start date	Start date
Finish-to-finish	Completion date	Completion date
Start-to-finish	Start date	Completion date

Note: For more information on finish-to-finish relationships and how schedule entries are created, see Task Splits and Scheduling Relationships.

Multiple Relationships

When multiple relationships are defined for a task, the start or completion date of one of the predecessor tasks is used to set the task’s start or completion date. Which predecessor task is used depends on the types of relationships created for the successor task.

Multiple Relationships of the Same Type

If there are multiple relationships of the same type, the rules defined in the table below are used to select the predecessor task that is used to set dates on the successor task. When a lag (or overlap) is specified on the predecessor, the lag time is added to (or subtracted from) the predecessor’s completion date before determining which predecessor to use.

Relationship Type	Predecessor Used	Successor Date Set
Multiple Finish-to-start	Completion date of the latest finishing predecessor, plus the lag or minus the overlap	Start date
Multiple Start-to-start	Start date of the latest starting predecessor, plus the lag time or minus the overlap	Start date
Multiple Finish-to-finish	Completion date of the latest finishing predecessor, plus the lag time or minus the overlap	Completion date
Multiple Start-to-finish	Start date of the latest starting predecessor, plus the lag time or minus the overlap	Completion date

Multiple Relationships of Different Types

When multiple relationships of two or more types are defined for a task, the successor task’s start and completion dates are calculated as follows:

•	If lag or overlaps have been defined for any of the relationships, the predecessor’s date is offset by the lag or overlap.

•	If finish-to-start or start-to-start relationships exist, the start date is set to the latest of:

•	The completion-date from the latest finishing FS predecessor, or

•	The start-date from of the latest starting SS predecessor.

•	If finish-to-finish or start-to-finish relationships exist, the completion date is set to the latest of:

•	The completion-date from the latest finishing FS predecessor, or

•	The start-date from of the latest starting SS predecessor.

•	If only FS and SS relationships exist, the successor’s dates are calculated as follows:

•	The start-date is set to the date calculated above for the FS/SS relationships

•	The completion-date is calculated by adding the task duration to the start date

•	If only FF and SF relationships exist, the successor’s dates are calculated as follows:

•	The completion-date is set to the date calculated above for the FF/SF relationships

•	The start-date is calculated by subtracting the task duration from the completion date

•	If combinations of FS/SS and FF/SF relationships exist, the successor’s dates are calculated as follows:

•	The start-date is set to the date calculated above for the FS/SS relationships

•	The completion-date is set to the later of start-date plus duration, or the completion-date calculated for the SF/FF relationships