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Microsoft Project 2010 : Linking Tasks (part 1) - Defining Dependency Links

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As you know, projects are made up of tasks. Together, these tasks should represent the entire scope of work that needs to be completed in order to successfully finish the project. In that sense, all tasks of a project are connected; they all contribute to the successful completion of the project.

However, some tasks are linked to each other in greater detail. There are two types of tasks when working on a project: summary tasks and subtasks. Summary tasks provide summary information about the tasks that are subordinate to the summary within a project, and the subtasks are the detailed tasks that must be accomplished to complete the summary tasks. For example, if you had a project called Home Move Plan, a summary task would be Packing, and subtasks would be Inventory and Organize All Possessions, Clean Closets and Storage Areas, Conduct a Sale or Make Donations, and so on. The subtasks are detailed tasks of the summary task, and they can (and should) be logically linked to each other. The total span of time, from the earliest start date to the latest finish date, for all the subtasks is the duration of the summary task. These types of relationships between tasks, and how they apply to Project, will be discussed in the following sections.

Understanding Task Relationships

Some tasks in a project often cannot start until one or more other tasks have finished. Usually this is because a task might need to use the output generated by another task. In the example of moving to a new house, you must calculate your expenses before you can determine the best method of moving. Therefore, the start of the task Determine the Best Method of Moving is determined by, and should be linked to, the finish of the task Calculate Moving Expenses. The link expresses that the schedule for determining the best moving method is dependent on the schedule for calculating your expenses, because you cannot make the decision until you know your moving budget.

A novice scheduler in this moving project might just list the tasks and type in start and finish dates for all the tasks using Manually Scheduled mode. But if the scheduler later finds out that for some reason the Calculate Moving Expenses task will be delayed, he or she will then have to go through the entire project, typing in later start and finish dates for all the tasks that will be affected by the initial delay; an inefficient and error-prone process. When project schedule tasks start or finish dates are manually entered on an auto-scheduled task, a Constrained Task symbol is added to the Indicator column. Constraints are not added to Manually Scheduled tasks.

However, if the auto-scheduled tasks are created in Project and dependency links are defined, the scheduler can simply enter a delayed start date for Calculate Moving Expenses, and Project calculates the new start and finish dates for all the tasks that are dependent, directly or indirectly, on that task. To correct the schedule due to a delay, all the scheduler has to do is modify the task (the start date or the duration), and all the other tasks will be recalculated and those changes highlighted. The reason you take the time to define links between tasks is so that Project can recalculate the schedule for you quickly when there is a schedule change that affects other tasks.

Defining Dependency Links

Tasks are linked to show a dependency relationship as predecessor and successor tasks. The predecessor task determines the schedule of the successor, or dependent task. The terms predecessor and successor are used to refer to the direction of the link and not the position of the tasks. You will see that more sophisticated linking can create a situation where successor tasks begin before predecessor tasks are accomplished, even though the successor tasks depend on the predecessor tasks for completion. 

To illustrate usage of the terms successor and predecessor, suppose you need to schedule the painting of a room. Four tasks are involved: Put on the Protective Tape, Apply the Primer, Apply Paint, and Clean Up. The start of Apply Paint depends on the finish of Apply Primer; therefore, Apply Primer is the predecessor to Apply Paint. Similarly, Apply Paint is the predecessor to Clean Up (because you cannot clean up until you have applied the paint). In theory, Clean Up is the successor to Apply Paint, because you want to clean up after all the painting is done, but realistically you can begin to clean up before completing the Apply Paint task. 

The start date for the successor task should be linked to the finish date for the predecessor. As illustrated in Figure 1, Microsoft Project draws a small arrow from the finish of each predecessor task to the start of its successor task.

Figure 1. Predecessor and successor task relationships are indicated by arrows drawn from the predecessor taskbar’s start to the successor taskbar’s start.

When you refer to a dependency link, the linked date of the predecessor task (either its start or its finish) is named first, and the linked date of the successor task is named last. In the painting example in Figure 1, the dependency relationships for the first three tasks are called Finish-to-Start links because the predecessor’s finish determines the successor’s start. Finish-to-Start is the most common type of link, but there are three other types you can use: Finish-to-Finish, Start-to-Start, and Start-to-Finish. 

By establishing the link in the painting example, you instruct Microsoft Project to set the start date for Apply Paint based on the scheduled finish date for Apply Primer. Any change that alters the calculated finish date for the predecessor causes Project to also reschedule the start date for the dependent or successor task. If you define task links, Project automatically reschedules dependent tasks when the schedule for the predecessor changes.

When you link two tasks in manual task mode, Project shifts the start date of the successor task to the finish date of the processor task as if the tasks were auto-scheduled. However, changes to the predecessor task will not automatically propagate to the successor. The successor tasks will be highlighted with a dashed “warning” outline, indicating that the task would move if it were auto-scheduled.

Caution

Do not link two unrelated tasks just to level out the workload for a resource who is assigned to work on both tasks. It is true that the link forces Project to schedule the tasks one after the other, thus allowing the worker to complete one task before starting the next. But if the worker is later removed from working on one of the tasks, there is no way to tell that the link no longer serves a purpose and can be removed (unless you just happen to remember it or create a note on the task), and you will be left with an unnecessary delay that could prolong the finish of your project. The preferred way to deal with this problem is to delay one of the tasks by using the Leveling Delay Field.


Defining the Types of Dependency Link Relationships

You can create four types of dependency relationships, depending on whether you use the start dates or finish dates when linking tasks. The name for each dependency type includes a reference to the linked date for the predecessor (either its start date or its finish date), followed by a reference to the linked date for the dependent task (either its start or finish date). Therefore, a Finish-to-Start relationship signifies that the finish date of the predecessor task is used to schedule the start date of the dependent task. The predecessor is referenced first, and then the dependent or successor task is referenced.

Project usually uses two-letter code abbreviations for the four dependency types, as shown in Table 1. The first letter in the code refers to the predecessor’s start or finish, and the second letter refers to the dependent task. Thus, the code for Finish-to-Start is “FS.” The following subsections describe the different dependency types.

Table 1. The Linking Relationships in Microsoft Project
CodeDependency TypeMeaning
FSFinish-to-StartPredecessor’s finish determines successor’s start.
SSStart-to-StartPredecessor’s start determines successor’s start.
FFFinish-to-FinishPredecessor’s finish determines successor’s finish.
SFStart-to-FinishPredecessor’s start determines successor’s finish.

By default, when you link tasks, they are defined as a Finish-to-Start relationship. You can change this dependency relationship type in the Task Information dialog box. After you have established the link, open the Task Information dialog box for the successor task by right-clicking on the task and selecting Task Information. Then, on the Predecessors tab, you can change the dependency relationship from the drop-down list in the Type column. Be sure you choose correctly; remember that the predecessor comes before the successor in the name (Start-to-Finish means the predecessor’s start determines the successor’s finish).

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