When you assign or remove people from a task, Project lengthens or shortens the duration of the task based on the number of resources that are assigned to it, but Project does not change the total work for the task. This is called effort-driven scheduling and is the setting Project typically uses when you assign resources to tasks.
Double-click the task.
In the Task Information box, click the Advanced tab.
Uncheck the Effort driven check box.
You can add the Effort Driven column to a sheet view. Right-click a column heading and then click Insert Column.
Although effort-driven scheduling works most of the time, you may want to change this to more accurately reflect what happens when resources are added or removed for a particular task. For example, you may want to see the total work increase as you add more people to a task.
Here are some exceptions to watch out for when you change the effort-driven setting.