Microsoft Project – Fixed Duration, Fixed Units and Fixed Work
One of the more difficult scheduling aspects in using Microsoft Project to schedule effort driven work is the concept of Fixed Duration, Fixed Units and Fixed Work. I have heard numerous colleagues including myself at times say “what’s wrong with Microsoft Project, its changing the schedule in crazy ways when resources are added or subtracted”. In fact, Microsoft Project is functioning correctly once one understands the way fundamental way it works.
Microsoft Project, when scheduling tasks based on effort driven schedules allows the project manager to work with three different variables. The project manager can only control two of these variables while Microsoft Project always automatically calculates the third. These variables are:
1. Fixed Units
2. Fixed Duration
3. Fixed Work
It is critical for a project manager to have a comprehensive understanding of the implications of each of these variables.
Fixed Units suggests that are the amount of capacity that a resource can devote to a task. An example of this is that you suggest to Microsoft Project that a resource can only work 50% of the time on a specific task. Microsoft Project will then automatically calculate the duration of the task with the resource only working 50% of the time.
Fixed Duration suggested that the task the task must be completed within a given duration. As you assign a single or multiple resources to the task, Microsoft Project will automatically calculate the appropriate resource allocation percentage to ensure the task is completed within the given duration.
Fixed Work suggests that a task has a specific numbers of hours work associated with it. In this scenario, we know that the task is going to take ten hours to complete. We have the ability to schedule the tasks overall duration for five days with fixed work of ten hours. Assuming a single resource is responsible for completing the task, Microsoft Project will schedule the resource to work two hours a day for the five day duration.
There are several advanced scheduling scenarios that can influence what is discussed above including front, middle and back loading of resources which aren’t addressed in this blog.
Dan Thompson, PEI
Great efforts, I appreciate.
Excellent article – crisp and clear, very useful
This glosses over the fact that Fixed Duration implies that MS Project will treat the duration as fixed, and will not adjust the duration. That is in fact not true. There are many situations where MS Project will change the duration even if the task is set to Fixed Duration. For example, if at some point in time the Remaining Work is high enough that the work cannot be completed in the remaining time, i.e. the calculated % (Units) will be above the Max Units for the Resource or 100%, then MS Project will change the Duration. Sometimes it will even change the Duration if the % needed to complete the Remaining Work in the remaining time is above the Unit currently set or calculated by MS Project. MS Project has a strong bias to keep the Units fixed even if the task is not set to Fixed Units, and will often adjust the Duration even if the task is set to Fixed Duration.