One feature that even experienced Project users sometimes have trouble with is task types. There are only three task types, but their impact on task scheduling is not always easy to predict. This week I look at task types: fixed duration, fixed units, and fixed work, and how they affect the scheduling of tasks.
Tell Me About Your Childhood
To understand why the task types feature exists at all, you first need to know about the scheduling formula. The scheduling formula is, quite simply, the simple little math formula that makes Project an active scheduling engine and not just a spreadsheet plus Gantt Chart drawing tool.
As you may recall from previous blog posts, the scheduling formula is:
Work = Duration x Assignment Units
Applying your high-school algebra, this leads us to the following:
Duration = Work / Assignment Units
Assignment Units = Work / Duration
These Rules Were Not Made To Be Broken
Unless you turn off automatic scheduling, Project is always calculating the scheduling formula for every task and every resource assignment in your project plan. (This is true for 2007 and earlier at least; Project 2010 introduces a different concept that we'll examine in a future blog post). Because you are free to change any task's or any assignment's duration, work or units value, Project needs a basic set of rules to follow about how it should respond to your changes. That's where task types come in.
I've met plenty of people who get stuck on the "fixed" in the task type names (fixed duration and so on). What "fixed" means in this context is this: for any given task type, you're free to change any variable of the scheduling formula you wish (work, duration, or units) but Project will not change the variable that is fixed by the task's task type: work, duration, or units. In other words, "fixed" in the task type means fixed (or unchangeable) only to Project. Let's look at an example.
Not A Story Problem
Here's my initial task list.
TIP Click the screenshot image to see a larger view.
Note that I'm showing the task list with the Task Form displayed; this is one way I can see duration, work and units all in one view.
All of these tasks are of the fixed units type, which is the default task type in Project. For a fixed units task, I (the user) am free to change any scheduling formula variable I wish and Project will follow a set of rules in how it responds to keep the scheduling formula accurate. Let's focus our attention on task 3. Currently the scheduling formula for task 3 is:
160 hours Work = 2 weeks Duration x 200% Assignment Units
Converting the work and duration values to common time increments, I get:
160 hours Work = 80 hours Duration x 200% Assignment Units
Note that the duration is always the task's duration while the work and units values are the sums of those values per resource assignment. For example the 160 hours of work above is the sum of the Screenwriter's 80 hours work plus the Artist's 80 hours work.
Now I'll change the task type of task 3 to fixed duration. (I do so through the Project menu, Task Information dialog box, Advanced tab. Or I can do so right in the Task Form.) There's no immediate impact on the schedule; Project accounts for task type only when rescheduling a task (or assignment). Watch what happens though when I change each of the scheduling formula values this fixed duration task. First I increase duration from 2 to 3 weeks:
Project recalculates Work. The updated scheduling formula is:
240 hours Work = 120 hours Duration x 200% Assignment Units
I'll undo that change, and this time I'll change Assignment Units from 100% to 50% for each of the two assigned resources:
80 hours Work = 80 hours Duration x 100% Assignment Units
There's no action I can take to cause Project to recalculate duration though, because this is a fixed duration task.
I leave the exercise of testing the other task types to you, dear reader. The essential thing to know is that the task type gives Project the rule set it needs to keep the scheduling formula accurate as any scheduling formula variable changes in a project plan.
Hands-on with Project Step by Step
To read more about this blog entry's subjects in the two most recent editions of Tim Johnson's and my Project Step by Step books, see the following cross-references.
Changing task types
- Project 2007 Step by Step: "Changing Task Types," pg. 156.
- Project 2003 Step by Step: "Changing Task Types," pg. 140.