The rollup options for a custom field define how Project will populate the values in a custom field for summary task rows. If you choose to have the values rolled up, you can choose from Average, Average First Sublevel, Maximum, Minimum, or Sum. The Budget and Remaining Budget fields are examples of the use of rollup options. To get a real representation of summary task budgets , we want the Budget field rollup option to be set to Sum so that the summary task value for the Budget field will be the sum of all the subtasks ' Budget fields. The same would go for the Remaining Budget field. Having the formula determine the summary task value tells Project not to rollup the subtask values, but to instead calculate the summary task field values based on the formula. For the Remaining Budget field, this option works as well as the Sum rollup option.
Setting Rollup Options
Click Tools, Customize, Fields from the menu bar.
To select rollup options, click the Rollup radio button.
Click the drop-down arrow for the Rollup field and select the desired option.
To select a formula, select the Use Formula radio button.