Project Management: Difference between Fixed Unit, Fixed Duration and Fixed Work

When creating a task in Project.net you have the choice of fixed units, fixed duration or fixed work. Most use fixed work tasks, but what exactly do these terms mean? Hopefully, this post will shed some light.

Fixed Unit tasks are used when the % work assigned (units) to a resource needs to be always be the same regardless of the duration and work required. If the assigned task type is Fixed Units / Effort Driven, assigning additional resources shortens the duration of the task.

Fixed Work tasks are used when the work required for the task is constant and the duration and resources (hence units) assigned to the task are flexible. If the assigned task type is Fixed Work, assigning additional resources shortens the duration of the task. Fixed work by default means it is effort driven so that option has not been made available when a task is assigned to be FW.

Fixed duration type is assigned when the duration of the task needs to be constant regardless of the units and work. If the assigned task type is Fixed Duration, assigning additional resources should decrease the amount of work for each individual.


Summary of Task Types

		Assignments	duration	work
Fixed Unit	Effort Driven	never being changed changing, adding or removing assignments changes duration.	Changing duration changes task work.	Changing work changes duration.
	Non-Effort Driven	never being changed changing assignments changes duration.	Changing duration changes task work. adding or removing assignments changes task work.	Changing work changes duration.
Fixed Duration	Effort Driven	Changing assignments changes task work. Adding or removing assignments changes the amount of time resources spend working.	never being changed Changing duration changes task work.	Changing work changes the amount of time resources spend working.
	Non-Effort Driven	Changing, adding or removing assignments changes task work.	never being changed Changing duration changes task work.	Changing work changes the amount of time resources spend working.
Fixed Work		Changing, adding or removing assignments changes duration.	Changing duration changes the amount of time resources spend working.	never being changed Changing work changes duration.

Source: http://forums.project.net/showthread.php?t=612

MS-Project uses the following formula to calculate duration, resources (units) and effort.
Duration X Units = Work

To create a fixed Duration task:

1. Insert the Type field into the Gantt Chart view
2. Enter the task name
3. Change the Type field to Fixed Duration
4. Enter the Duration
5. Enter the Resources
6. Effort will be calculated by MS-Project

To create a fixed Work task:

1. Insert the Type field into the Gantt Chart view
2. Enter the task name
3. Change the Type field to Fixed Work
4. Enter the Effort in the Work field
5. Enter the Resources
6. Duration will be calculated by MS-Project

Source: http://www.tacticalprojectmanagement.com/microsoft-project-tips/microsoft-project-tip-3-use-fixed-duration-fixed-work-and-fixed-unit-type-fields.html