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Do’s and Don’ts: Using Hammock Tasks

girl-1632836_1920Every so often I’m asked to reproduce a project management technique in Microsoft Project that isn’t a named feature in the software. Such is the case with “hammock” tasks.

A hammock task is a task that has a variable duration. The duration is controlled entirely by other tasks in another or the same project. The start and finish of the hammock is linked to the other tasks’ start or finish. The duration of the hammock is recalculated when the linking tasks change their dates.


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Opinions about hammock tasks range from “Don’t do it!” to “Always drive your level of effort tasks with the technique.” It’s easy to set up a hammock task; but there are caveats that you need to know before using the technique in a production environment. I’ll cover some of those in this article as well.

A hammock task looks pretty inconspicuous:

Sam_Huffman_Hammock_tasks_figure_1

The task names point out their function. The start of the hammock is the paste linked start date from “Hammock Start Link,” which I’ve formatted in bold red in the figure above. The finish of the hammock is the paste linked finish date from “Hammock Finish Link” formatted in bold green. Note the indicators in the lower right of the hammock tasks date cells. These indicators are visual markers telling us the dates are linked.

I will double the duration of task #2. This forces a recalculation, and the duration of the hammock task is changed to six days. The calculation may take a few seconds, even on a very fast machine, so be patient.

Sam_Huffman_Hammock_tasks_figure_2

I wrote this article using Project Professional in Manual Schedule mode, with a resource assigned at half time. This worked well with a simple resource using the same calendar as the project and with no overallocations. It’s likely that I would have to manually resolve any task or resource scheduling issues in a more complicated project. This would be true regardless of the scheduling mode selected.

Now for some caveats:

  • Hammock tasks can be confusing to anyone reading your schedule. So include a note restating the purpose of the task. For example, is it to collect a level of effort for management resources? Synchronizing the project with other tasks or projects in a program schedule?
  • Paste links can be lost easily, so visit the task and its position in your schedule often.
  • Since a hammock task receives its dates as a paste link, the task probably shouldn’t have a predecessor or successor. If it’s necessary to link with other tasks, revisit your need for a hammock task. Maybe a normal task will meet your needs.
  • Hammock tasks are schedule-driven and not resource-driven. Be cautious when leveling resources. You might consider making the task a higher priority than other tasks so Project will skip the task when leveling the rest of the project.

There are always sequencing workarounds and alternatives, but the hammock task continues to be one of the primary tools used in scheduling. You can find more on this subject online and in MSDN. Use the term “hammock task” and you’ll get a great deal of information on the pros and cons of using a hammock task.

A version of this article originally appeared on Sam Huffman’s “Project Blog” here.

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9 Comments
  1. Great summary of pros, cons and caveats. One more caveat I’d add is the confusion that can be caused by a hammock task appearing to be critical. For a project management task as a hammock between Kick-off and Final close-out, the total slack would be 0. One could get round this – if one saw it as a problem – by making the finish hammock milestone have a slight lead time against the last project event.

    Reply
  2. Great blog,
    I use hammock tasks, but prefer to use driven start and finish milestones within a summary task to provide a hammock (summary) task.
    Can you see any pros, cons or caveats with this method?
    Cheers, Mark.

    Reply
  3. Do not use these in Project Online or Project Server if you intend to open and save the schedule using the browser.

    Reply
  4. Thank you Graham, Mark and Gary,

    In your three comments you can see why this technique is controversial! Additionally, 2 alternatives have been presented for reader consideration. There is no doubt of the need for hammock tasks, but how they can be implemented and utilized is the question.

    Maybe there are other hammock task techniques the readers would like to comment on?

    Reply
  5. I find them useful and frustrating. I’m using a master/sub schedule on Project Server which breaks the links every time I save the master. To recover, I always put the unique ids of the start and finish tasks in the notes. I also attach the prefix HMK: on the task name.

    For the few reports I use with the hammocks, re-establishing the links is easy.

    Also, remember not to use fixed duration tasks for hammocks.

    Reply
  6. Here is another reason not to use this technique with Project Server as Gary already mentioned; if you combine this cell linking technique to create hammock tasks with the My Timesheet or My Task feature in PWA you will get into a real mess.

    Reply
  7. I often use hammock activities to report the overall content and status of my time schedules, for instance at management level in the organisation.
    I wonder, however, how do I track the activities linked to a hammock activity if I want to go back to verify if the starting or finishing activities are still applicable?

    Reply
  8. Using a hammock task causes a warning message every time the schedule is opened, prompting to update links. This is annoying, and causes confusion for others who look at the schedule. Is there a way to stop this message and just update automatically?

    Reply

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