PRJ: How Elapsed Duration Tasks Affect Slack Calculations

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Article ID: 87301 - View products that this article applies to.
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Tasks with elapsed durations, rather than working durations, may introduce slack into a project because these tasks can extend into nonworking portions of the project calendar.


Setting an elapsed duration on a task (for example, "1ed" as opposed to "1d") fixes the duration of the task in an absolute sense. An elapsed duration allows Project to extend the task into nonworking portions of the project calendar. A "4eh" (4 elapsed hours) task that began at 2pm, for example, would finish at 6pm without regard to the project calendar. This may result an unexpected amount of slack for some tasks.


NOTE: For this example, use the Standard project calendar.

  1. Create two tasks; task 1 with a duration of 8h, and task 2 with a duration of 9eh.
  2. Create a third task, task 3, with a duration of 1d.
  3. Set the project start date to 7/24/92 8am.
    Project 2010: On the Project tab, click Project Information.
    Project 98-2007: On the Project menu, click Project Information.

    Project 4.1 On the File menu, click Project Info.

    Project 4.0: On the File menu, click Summary Info.
  4. Make tasks 1 and 2 predecessors (FS) of task 3.
In this example, both tasks 1 and 2 start on 7/24 8am, and finish on 7/24 5pm; yet the Total Slack for task 1 is 0d, while the Total Slack for task 2 is 2.63ed.

Because task 1 has a working duration of 8h, no portion of that task can be scheduled between 7/24 5pm (Friday) and 7/27 8am (Monday); hence, it has no slack.

However, task 2 has an elapsed duration of 9eh, and so could be scheduled to start as late as 7/28 11pm (Sunday) and still not delay task 3. The Total Slack is the difference between 8am Friday and 11pm Sunday, 2.63ed (63h).

Note that elapsed duration tasks are scheduled to start just as any other task; that is, they are scheduled to start only during calendar working hours. The elapsed duration implies only that the task can extend into nonworking hours once having started. For task 2 to start between 5pm Friday and 8am Monday, it would have to be forced with either a Constraint or an Actual Start/Finish date.


Article ID: 87301 - Last Review: February 27, 2014 - Revision: 5.0
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