Estimating a project's earned and final duration

Using standard Earned Value Management (EVM) protocols, the current approach to Earned Schedule (ES) is extended and formalized to establish its rigorous, theoretical foundation. A precise definition is provided for what we term the project's earned duration, whose creation completes the triad of planned, actual, and earned durations. The published ES formula emerges as a linear approximation, but is found to work with some nonlinear cost profiles, and the conditions under which it gives both correct and incorrect duration estimates are noted. In the several planned and earned value functional profiles examined, no approximations are required to derive an exact analytical expression for the final duration; most duration formulas are straightforward and useful. The reliability and accuracy of the duration formulas are demonstrated with several examples of real, nonlinear project data that represent large classes of projects. We conclude with practical guidance for project managers.