Effort-driven scheduling, editing existing assignments, and resource availability all can affect the way Microsoft Project schedules.

Effort-driven scheduling

The way that assigning resources to tasks affects the schedule depends on whether the tasks are effort-driven.

Effort-driven scheduling indicates that as you add resources to a task assignment, work is redistributed among all the resources to maintain the same amount of work overall for the task. Likewise, for effort-driven tasks, if you remove resources from a task, work is redistributed among the remaining resources, again, to maintain the same amount of work overall.

Keeping the amount of work constant in effort-driven scheduling can have varying effects on duration and units, depending on the task type. By default, tasks are fixed-unit, effort-driven. This means that as more resources are assigned to a task, the less work each resource needs to do. Because it's a fixed-unit task, its duration is also reduced.

If you have a fixed-duration, effort-driven task, the more resources you assign, the less work each resource needs to do to complete the reduced amount of work for the assignment. Also, because it's a fixed-duration task, each resource can put in less effort, that is, fewer assignment units.

By its nature, a fixed-work task is effort-driven. Just like with fixed-unit, effort-driven tasks, if you add resources, the work is distributed among the assignments and duration decreases.

Editing existing assignments

After you assign resources to tasks, task scheduling is affected primarily by the assignment units and resource calendars. If you later edit these assignments, you can affect the task scheduling, which is influenced by the relationships of assignment units, work, and task duration.

Specifically, the way scheduling is affected depends on the task type. There are three task types: fixed unit, fixed work, and fixed duration.

For all tasks, after you assign a resource, the task is scheduled according to the formula Duration = Work / Units. The following table shows how each of the task types affects scheduling when you edit one of the three elements.

For variable rate material resources, if you change units in a fixed-work task, work (instead of duration) is recalculated.

If you revise units, work, or duration on an assignment, one of the other two items is fixed according to its task type, and therefore Microsoft Project recalculates the third item.

For example, in a fixed-unit task, if you change the amount of assigned work, the task duration changes, while the assignment units remains fixed. Likewise, if the task duration changes, this causes a corresponding change to work. Set a task to fixed unit when the assignment units should not change, even if the duration or work amount were to change. For example, you set a resource to 50% units on an assignment, and you never want that unit value to change, even if it means the task duration or the work increase. Fixing the assignment units is appropriate when the resource availability on the project, and therefore on the assignment, is inflexible. This is the Microsoft Project default.

Resource availability

Resource availability is based on resource start and finish dates on the project, as well as the resource's maximum units. While availability does not initially affect task scheduling, if a resource is scheduled for tasks beyond his or her available dates or maximum units, the resource will be marked as overallocated. If you then level the resource, or make other adjustments to resolve overallocations, assigned tasks can be split or delayed, and that changes the task scheduling.

If a resource doesn't start or finish at the same time as the project start and finish date, you can specify the resource's starting and ending availability dates in the Resource Information dialog box.

You can also specify the resource's maximum units for the project as a whole: full-time (100%), part-time (50%, for example), or multiple (for example, 300% for three carpenters on the project). In the Resource Information dialog box, you can also specify that a resource unit availability varies for different time periods throughout the project. For example, use this table to indicate that three painters (300%) will be available from June through August, and one painter (100%) will be available September and October.

If the work assigned to a resource is outside the resource's start and finish dates or available time, or if it exceeds the resource's maximum units, Microsoft Project assigns the resource but indicates that the resource is overallocated. You can then be alerted to the potential problem and decide how to fix it.