Lesson 2: Link Tasks

  
 
In most cases, the tasks in a project are related to each other, and the relationships between them drive the schedule for the project. The relationships between the tasks are called "dependencies." Typically after the tasks were inserted in the project and the WBS structure was created, the next step is to link the tasks among each other, to show relationships between them.

While building a project plan, you may encounter situations when a task can start only after another one is completed. I.e., you want to furnish the room only after the walls are completely painted.

Tasks scheduling is performed based on duration estimates of each individual task and tasks interdependencies that constrain a task to start only after other tasks in the project have been completed.

After introducing the WBS and duration estimates for each task, you have to introduce the interdependencies among tasks.

The constraint in the example above (furnishing after painting), corresponds in Project Plan 365 to a Finish-Start constraint.

Other constraints can be imposed in Project Plan 365 as well. I.e. for building a foundation you may start leveling the concrete once the concrete started to be poured. This corresponds to a Start-Start constraint.

Such constraints that involve two tasks and condition the start/finish date of one task on the start/finish date of the other task, are imposed in Project Plan 365 by creating links between tasks. By linking tasks Project Plan 365 adjusts the schedule whenever there are changes that affect d uration of other tasks.

A task that needs to be completed before are called predecessor task and the linked tasks are its successors.

The link types supported in Project Plan 365 are summarized below:

   1. Finish - Start (FS) - the default link, forces a task to start only when/after its predecessor has finished; predecessor finishes and the other starts. A link can also have a "lag" value specified. Finish-to-start is the most common dependency type. If a nonzero lag is present, it is added to the Start date; otherwise the task starts immediately after the predecessor is completed. A positive la value will create a gap between the finish of one task and the start of another.

Task links           The dependent task (B) cannot begin until the task that it depends (A) on is complete.

   2. Finish - Finish (FF) - forces a task to finish only when/after its predecessor is completed; both tasks finish at the same time. If a nonzero lag is present, it is added to the Finish date. For instance a value of 4FF+2 days in the Predecessors column signifies a Finish - Finish type link with a lag of two days, where the current task has the task with ID = 4 as its predecessor.

Task links           The dependent task (B) cannot be completed until the task that it depends on (A) is completed.

   3. Start - Start (SS) - forces a task to begin only at the same moment with its predecessor or after its predecessor started; start of the predecessor determines when the other starts. If a nonzero lag is present, it is added to the Start date.

Task links           The dependent task (B) cannot begin until the task that it depends (A) on begins.

   4. Start - Finish (SF) - forces a task to finish only when/after its predecessor start. If a nonzero lag is present, it is added to the Finish date.

Task links           The dependent task (B) cannot be completed until the task that it depends on (A) begins.

You can link two tasks by one of the following options:

• In the column Predecessors enter:

   • the ID of the predecessor; this automatically creates a Finish-Start link from the predecessor to the current task

   • the ID of the predecessor followed by the link type - a value in {SS, SF, FF}

• Using a vertical drag-and-drop operation starting from the predecessor: implicitly a Finish-Start link is created; to change the link type or to delete it, perform a double click on the link.

• Double click on any task to bring up the Task Information dialog. Select the Predecessors tab and enter the predecessor for the selected task. Also by using the Task Information dialog you can review the dependencies a task has and change if you want the type of link between two tasks.

icon_exercises.png  Avoid using too many complex links; it makes the project less manageable and the schedule more difficult to interpret.


 

icon_exercises.png           Practice

1. Double-click on task with ID = 59 (Install conduit at ceiling plenum space) and using the Predecessors tab from the Task Information dialog, from the drop-down list set its predecessor to be task with ID = 58 (Paint walls and woodwork) and add a positive lag of 4 hours. ​

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2. Using the Predecessors column in Gantt Chart view, introduce the following links between tasks. Notice how different types of links between tasks are depicted in the Gantt panel.

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When hovering the mouse over the separating line between the left side grid and the right side panel in Gantt Chart, the mouse pointer will indicate that a resize is possible. This is a convenient way to show more content from the right side Gantt panel.
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3. Notice the new duration of the first summary task called 'Three-story Office Building (76,000 square feet)' and observe how the introduction of dependencies between tasks affects the duration of summary tasks.