13.3. Preserving the Original Plan in Project

After stakeholders approve a project plan, they expect the project to follow that plan. If it doesn't, the stakeholders expect an explanation of the difference between planned and actual performance (called variance). Variance is the foundation to a variety of project performance measures (Section 15.5.1), like earned value and the schedule performance index. To calculate variance, you must first save the original planned values in your Project schedule.

As you've seen in previous chapters, Project constantly updates fields like dates, duration, work, and cost as you make changes (as long as tasks are set up as automatically scheduled). For example, if you increase the work hours for a task, Project recalculates the duration and the finish date for the task. When you begin to track progress, Project continues these recalculations; for example, as a delayed predecessor task pushes back the start dates for its successors.