Cost/Schedule Control – forecasting total duration
You can guess the next question, can’t you? Yes, that’s right: “When will the project be done?” The SPI (alternate) is the best (and only) predictor of total task time. Project Management Institute (PMI®) in the PMBOK® Guide (3rd Edition) has not yet committed to a standard for estimating completion time.

The forecast duration for the total project depends on the schedule performance of tasks on the critical path. You must also examine whether delays on non-critical path tasks and their new estimated durations will shift the critical path.