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The Magic Formula and Manual Work Assignment

We all know that the heart of MS Project is the “magic” formula: Work = Duration * Units. This formula works with automatically scheduled tasks, but does it always work? Let me illustrate my point by creating a simple project with two tasks:

As you can see, Work is 0 hours, because there are no resources assigned to either task.

When I assign John and Mary to both tasks, I will get the following result:

Here we can see that formula works as expected. Both resources are going to work 100% (units) for two days (duration). That is (by default setup) 16 hours per person per task (eight hours per person per day per task).

Sixteen hours (Work) = two days (i.e. Duration = 16 hours) * 100% (for both John and Mary).

However, let’s say that I want to see something a little more complicated:

  • Task 1
    • John will work six hours on Monday and two hours on Tuesday
    • Mary will work two hours on Monday and six hours on Tuesday
  • Task 2
    • John will work two hours on Monday and six hours on Tuesday
    • Mary will work 6 hours on Monday and 2 hours on Tuesday

This scenario can be reflected in Project only if I manually adjust work per day like so:

Here, the “magic” formula doesn’t work anymore. Note John on Task 1.

Eight hours (Work) = two days Duration (16 hours) * 75% (Units). But, 16 hours * 75% = 12 hours, which is not a case here. That is not a bug. Since John works different hours per day, it is impossible for the software to calculate the correct units, in this case.

When you input a project scenario like this one, you will find that this formula doesn’t work! Before you give up all together, you should use the Task Usage View, like so:

At least, this symbol indicates that someone had rearranged the work period manually—something you will not see in Gant Chart View!

To conclude, if you want to change assignment per day, know that the “magic” formula will not work. As a final example, take a look at the figure below:

Duration is four days (32 hours), but John has to work four hours on the first day, three hours the second day, two hours on the third day, and finally one hour the last day. His max units (assignment) is different for each day, so we see that the Peak column shows only his maximum on first day, which is 50%!

Have you encountered this formula not working, too, when working with manual work assignments?


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1 Comment
  1. Daryl Deffler

    What you’ve illustrated is how MS Project is designed to work.
    When you, the PM, manually edit the resource load pattern, the MS Project formula no longer applies. Project assumes you, the PM are taking total control for that task/resource assignment. The Peak value represents the highest allocation over the course of the duration. If, in our example, John’s four hour work day was on the 3rd day, the Peak would still show 50% because that was the highest allocation during the task duration across John’s manually assigned hours.
    Bottom line, once you, the PM manually enter planned resource assignment hours as illustrated in this article, the formula is no longer used and you own it. You can however re-engage the formula by manually changing the resource load pattern back to Flat.


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