A common cause of frustration when assigning resources in MS Project is how the work is scheduled. You may find that after assigning a few resources the duration of the task increases, or maybe it reduces, or just doesn’t behave how you expected. This frustration stems from a lack of understanding of how task types work.
This article looks to demystify how task types affect a schedule as well as provide examples of situations when one task type is more appropriate.
The Underlying Formula
To understand task types you have to be aware of the underlying formula used by the scheduling engine in MS Project:
Work = Duration x Units, where:
- Work: represents amount of effort, being typically measured in number of hours
- Duration: represents the length of a task, typically measured in number of working days
- Units: represents the percentage of assignment of a resource, typically measured in %
The task type choices are:
- Fixed Units
- Fixed Work
- Fixed Duration
The ‘Fixed’ aspect refers to which variable (Units, Work, Duration) remains constant when the other two variables are changed.
Enter the duration, work or units estimate and fix that number by setting the Task Type accordingly.
Fixing the value prevents Project from changing it. Hence…
- If you enter a Duration estimate, set the Task Type to Fixed Duration.
- If you enter a Work estimate, set the Task Type to Fixed Work.
- If you enter a Units estimate, set the Task Type to Fixed Units.
You can then enter a second value and, using the formula, Project will calculate the third.
You can use the table below to understand how MS Project performs schedule calculations based on the task type and changing variables:
Work = Duration x Units
*If the units are recalculated, then you will not see a change to the schedule, unless you are viewing the Peak Units. Peak Units represents the maximum effort a resource is working on a given assignment in the current time period.
So, what are the best uses for the different task types?
Use Fixed Unit tasks in situations such as:
- When the number of resources you have for the task is the first or main thing you know
- When you can’t get more resources to do the work — your resources are fixed
- When you want to keep the resource working on a task at a certain percentage (e.g. Project Management will take 10% of the task duration)
Use Fixed Work tasks in situations such as:
- When the effort required is the first thing you estimate
- When the effort required is the easiest thing to estimate. Estimating effort is usually easier and more accurate than estimating duration.
- When your resources give you task estimates in work hours. For example, developers and engineers typically give work estimates in hours.
IMPORTANT: When working with Fixed Work tasks, the Effort Driven option will be enabled by default. When a task is effort driven, MS Project keeps the total task work at its current value, regardless of how many resources are assigned to the task. When additional resources are assigned, remaining work is distributed to them. More on Effort Driven tasks later!
Use Fixed Duration tasks in situations such as:
- When the duration is the first thing you estimate.
- If the duration stays the same when adding resources, such as training or backing up a computer system.
- When the deadline is so tight that it’s the primary driver for the duration of the task (e.g. legal and compliance projects)
- When the workload isn’t your problem, such as when external resources are consultants are used.
TIP: Try to be as consistent as possible in the use of the task types, ideally, sticking to just one type across the project.
How to Amend Task Type
One way to amend the task type is to…
1. Right click on the task
2. Select Information
3. Go to the Advanced Tab
4. Choose a Task Type
5. Click OK
What about Effort Driven tasks?
An effort-driven activity indicates that the amount of work required to complete the activity does not change regardless of how many resources are assigned. When you add or delete a resource assignment on an effort-driven task in MS Project, work is distributed equally among resources.
By default, all Fixed Work tasks are effort driven, whereby the more people you add, or the more hours they work, the shorter the duration.
Similarly, when you make a Fixed Unit task Effort Driven, the more people you add to the task, the shorter the duration.
In comparison, when you make a Fixed Duration task Effort Driven, the more people you add to the task will not shorten the duration. Instead the work will remain fixed and the resource units for each resource assigned will reduce to accommodate the workload within the same duration.
Now that you’re aware of task types, as well as how effort driven scheduling works, you won’t be caught out by the behaviour of the task when assigning resources. Just remember the underlying formula used by the scheduling engine and that you only ever need to change two variables for Project to calculate the third.
By Rachel Goodwin – Senior PPM Consultant, Wellingtone