Back to the Basics: Calculating Duration in Fixed Duration Tasks

My colleague approached me with an interesting question the other day.  Under specific conditions, he could make seemingly identical tasks appear with different durations.

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So the question is “why?”  Why do seemingly identical tasks have different durations.  First off, lets look at how duration is calculated…

Fixed Duration Activities Without Assignments

In the example below, I have three tasks.  Each task is a different type.

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You’ll note that the duration is the same.  Next, I will introduce an exception of one day in the Project Calendar.  In theory, this will push out the finish date for each task.

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The results appear as below:

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We see that the duration has not changed, even though the tasks now finish up on 6/13 instead of 6/10.  This is as expected, because the duration is calculated as the total number of working days, and excludes any nonworking days such as weekends or holidays.

Fixed Duration Activities With Assignments

Now let’s assign a resource to each of those tasks.

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Everything looks correct.  The resource is using the same Project Calendar, and therefore, we anticipate no changes.  Here’s where things get a bit trickier.  I am going to add a 1 day exception to my resource calendar.

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…and now my schedule looks like this:

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See how the duration shows differently, yet for all intents and purposes, the start and finish dates are identical?

The reason for that is that Fixed Duration activities ignore the resource calendar when calculating duration.  A Fixed Duration task calculates the difference between the start and end date using the Task or Project calendar.  In this case, since I haven’t applied a Task calendar, the calculation uses the Project calendar.

Fixed Unit and Fixed Work tasks on the other hand, calculate duration by counting how many days on work is actually assigned.

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Back to the Basics: Calculating Duration in Fixed Duration Tasks

2 thoughts on “Back to the Basics: Calculating Duration in Fixed Duration Tasks

  1. Good one. Question for you…As a general practice, which of the following task types do you generally suggest an IT Organization to use? To me, it always seemed that Fixed Duration Tasks would fit in nicely as most of the planning is done with specific timelines in mind…

    1. Well, there’s no one answer fits all. For me though, in the consulting world, I typically like to do my estimates in Fixed Work, and then after baselining, flip all of my tasks to Fixed Units for tracking. If the organization is not doing effort based scheduling, I usually just recommend Fixed Duration as it’s simplest and most intuitive for project managers just starting out in the scheduling world. I would expect there to be a wide range of responses though.

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