March 5, 2007 – Deliverable-based Project Schedules: Fixed Work Sidebar

Before we move on to discussing duration and adding resources there is a side step we need to take to discuss different task types. When you are developing your project schedule, most tasks should be set as Fixed Work. As effort is expended or added to the task this allows the tool to re-calculate the duration and show the new estimated completion date.

In MS Project there are 3 types of tasks: Fixed Duration, Fixed Units and Fixed Work. I’m sure other scheduling tools have similar terms. Units (U) refers to the percent a resource is assigned to the task (the productivity factor discussed above). Duration (D) is the number of days a task takes to complete. Work (W) is the amount of effort expended or to be expended on the task. The formula for calculating Duration is D = W / U (ex. 8 hours / 80% = 10 hours = 1.25 days). This equation and variables are used to determine when the task will complete.

“Fixing” one of these variables tells the tool not to allow it to change. If Duration is fixed and you add more Work, the Units will increase to balance the equation. In order to complete a 16-hour task in 1 day the resource will need to be assigned 200%. If Units are fixed at 100%, reducing the Duration of a 16-hour task from 2 days to 1 day will force the Work to 8 hours in order to maintain the equation. Neither one of these options helps you track progress.

Selecting Fixed Work is more realistic. If you have an 8-hour task scheduled for 1 day and you decide to extend it to 2 days you want the Work to remain at 8 hours, not increase to 16. The time you allowed for the completion of the task increased, not the effort to do it.

This is a big topic from project managers trying to figure out what MS Project is doing to them. They change a duration or end date and suddenly the estimated hours either explode or shrink to nothing. Very frustrating.

On the flip side, there are tasks that should be Fixed Duration. These tasks include support, status reporting and others that are based on the calendar rather than the effort. Although I have never used it, there is probably a good application for Fixed Units. If you have an example, add a comment to this blog and I’ll pass it on.

The task type becomes important as we move to estimating the duration.