Carl Chatfield here, taking a break from working on Microsoft Project 2010 Step by Step. 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.
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.