In Microsoft Office Project Professional 2007, project managers can visualize the impact of changes and trace back their steps. To turn the Change Highlighting feature on or off, click the View menu, and then click Hide Change Highlighting or Show Change Highlighting. While the feature is enabled, all levels of tasks affected as a result of a change are highlighted with a color as a visual indicator. Using this feature together with the Multiple Level Undo feature, project managers can do "what-if analysis" by trying a set of changes and then reversing unwanted changes. If further analysis of a task's schedule is needed, the project manager can use Task Drivers on the Project menu to determine the factors (such as task dependency, calendar constraints, schedule date, or vacation time) that are driving a task's start date.